State Key Laboratory of Bioinspired Interfacial Materials Science，Innovation Center for Chemical Science，College of Chemistry，Chemical Engineering and Materials Science，Soochow University，Suzhou 215123，China
The advancement of characterization techniques has emerged as a pivotal driving force in refining structural theories of condensed matter chemistry. The unique interaction mechanism between neutrons and atomic nuclei/unpaired electrons enables neutron scattering techniques to provide distinctive information on light elements， isotopes， neighboring elements， and magnetism， thereby establishing complementary advantages with conventional optical， X-ray， and electron characterization approaches. Recent progress in high-flux neutron sources and in situ experimental methodologies has significantly expanded the application scope of neutron scattering from fundamental physics to complex chemical systems， making it an indispensable tool for deciphering intricate microstructures/microdynamics and reaction mechanisms. Among various techniques， neutron diffraction is mainly used to achieve a precise determination of both local and bulk structures. Concurrently， neutron spectroscopy offers unparalleled insights into dynamic processes and thus is particularly valuable for studying chemical bond breaking/formation， molecular conformation， molecule/ion diffusion/transport， and so on. Other neutron techniques， such as neutron imaging and small-angle neutron scattering， demonstrate huge potential for providing characteristic mesoscopic and macroscopic information. This comprehensive review systematically examines recent advances in neutron scattering investigations of condensed matter chemistry， with case studies underscoring the irreplaceability of these techniques in elucidating structure-property relations in complex systems. Furthermore， we present current challenges such as flux limitations， time resolution constraints， and multimodal characterization integration. We also propose forward-looking perspectives on methodological developments and synergistic characterization frameworks. These discussions offer valuable theoretical references and methodological guidance for researchers pursuing multi-scale characterization in condensed matter chemistry.
Contents
1 Introduction
2 A brief overview of neutron scattering techniques
3 Neutron diffraction studies of condensed matter chemistry
3.1 Single-crystal neutron diffraction studies
3.2 Neutron powder diffraction studies
3.3 Neutron pair distribution function studies
4 Neutron spectroscopy studies of condensed matter chemistry
4.1 Inelastic neutron scattering studies
4.2 Quasielastic neutron scattering studies
5 Neutron imaging/scattering studies of condensed matter chemistry
Accurate understanding of condensed matter systems plays a decisive role in the rational design of complex functional materials. Unveiling the relationship between structure/dynamics and physical properties has always been a key scientific challenge for researchers in condensed matter chemistry. Advances in characterization techniques have played a crucial role in furthering the in-depth study of these scientific problems.
Neutron scattering techniques, owing to their unique interaction mechanisms with atomic nuclei and unpaired electrons, have transcended the limitations of conventional characterization methods such as optical, X-ray, and electron-based approaches. They have demonstrated irreplaceable advantages in detecting light elements (e.g., hydrogen and lithium), resolving isotopes, distinguishing neighboring elements, and elucidating magnetic behaviors. In recent years, neutron scattering has emerged as a crucial tool for investigating the structure and dynamics of complex chemical systems as well as reaction mechanisms. It has thus showcased unparalleled value in cutting-edge condensed matter chemistry research spanning energy, environmental science, and biomedicine. Moreover, with advancements in neutron sources, neutron scattering technology has made significant strides toward multi-physics field coupling, in situ real-time monitoring, and multiscale modeling integration. These breakthroughs have not only deepened the theoretical foundations of condensed matter chemistry but have also facilitated the rational design of novel functional materials. The “multi-dimensional visualization” capability of neutron scattering is increasingly becoming a core driving force behind disruptive technological innovations.
This paper, guided by cutting-edge issues in condensed matter chemistry, systematically elucidates the unique advantages of neutron scattering techniques in theoretical research. By integrating representative case studies, it explores how these techniques are revolutionizing structure–property relationship studies. Finally, we envision that the cross-disciplinary integration of neutron scattering with artificial intelligence and other advanced technologies will usher in a new era for the characterization of condensed matter systems—shifting from "static snapshots" to "dynamic holography."
2 Introduction to Neutron Scattering Techniques
Neutron scattering, as a core characterization technique for investigating the multi-scale structure/dynamics of materials, is physically grounded in the unique interaction mechanisms between neutrons and atomic nuclei/unpaired electrons. Depending on the degree of energy exchange, this technique can be systematically classified into three categories: (1) elastic neutron scattering (ΔE ≈ 0), primarily used to elucidate micro-, meso-, and macroscopic structures; (2) inelastic neutron scattering (ΔE ≠ 0), mainly employed to study excited-state dynamics; and (3) quasi-elastic neutron scattering (ΔE → 0), primarily utilized to probe the spatiotemporal evolution of diffusion processes.
Neutron scattering techniques possess a distinct advantage in the detection of light elements. Due to the non-monotonic relationship between neutron scattering cross sections and atomic number (Table 1)［1］, neutrons exhibit unique sensitivity toward light elements such as hydrogen and lithium. This characteristic renders neutrons an ideal probe for studying systems including hydrogen-bonded networks (e.g., ice crystal structures) and lithium-ion transport (e.g., solid-state electrolytes). Neutron scattering also offers isotope-resolving capability. Isotopes sometimes display substantial differences in their scattering cross sections (e.g., 1H/82.03 barn vs. 2D/7.64 barn). By designing isotopic gradient systems, it is possible to elucidate diffusion pathways, interfacial reactions, and other dynamic processes at the molecular level, particularly suitable for *in situ* analysis of multicomponent systems. Furthermore, neutron scattering can resolve magnetic structures. The magnetic scattering cross section is directly related to the atomic magnetic moment, making this technique a key tool for characterizing spin order (ferromagnetic/antiferromagnetic), magnetic phase transitions, and magnetoelectric coupling phenomena. In addition, neutron scattering enables deep, non-destructive probing. Leveraging the centimeter-scale penetration depth of neutrons (e.g., thermal neutrons penetrate aluminum by 90% at 1 cm), this method can monitor *in situ* real reaction processes within thick-walled reactors under high-pressure and low-temperature conditions; reconstruct the internal structure of functional materials such as porous media in three dimensions; and dynamically observe biomacromolecules in living organisms.
Contemporary neutron sources have undergone three generations of development: the first-generation reactor sources (e.g., ILL), the second-generation spallation sources (e.g., ISIS, SNS, J-PARC, CSNS), and the third-generation high-flux sources (e.g., ESS). Major facilities worldwide exhibit a regional clustering pattern, with Europe (e.g., ISIS, ILL, ESS), North America (e.g., SNS, LANSCE), and Asia (e.g., J-PARC, CSNS) forming three major R&D centers. Since its commissioning in 2018, China Spallation Neutron Source (CSNS) has achieved breakthrough results in fields such as lithium-ion battery materials and hydrogen storage systems. The Phase II project will double the beam power, injecting new momentum into China’s condensed matter chemistry research. Neutron scattering technology is driving breakthroughs across multiple disciplines: (1) in the energy sector, it elucidates dynamic processes in energy storage and catalysis; (2) in soft matter systems, it tracks the conformational evolution of polymer chains; and (3) in biomedicine, it reveals the mechanisms underlying active site functions. With the completion of megawatt-class high-flux sources (e.g., ESS), the time resolution of neutron scattering will be enhanced to the microsecond scale, opening up new opportunities for capturing transient reaction intermediates. It should be noted, however, that the full potential of neutron scattering technology remains constrained by several challenges: the scarcity of high-throughput experiments, the heavy reliance on theoretical models for data analysis, and the need for holistic, multiscale analysis of dynamic processes—challenges that urgently require synergistic advances in both technology and methodology.
3 Neutron Diffraction Studies in Condensed Matter Chemistry
Neutron diffraction techniques analyze material structures by utilizing the diffraction phenomenon that occurs when a neutron beam interacts with a sample. The advantages of neutron diffraction lie in its sensitivity to light elements and magnetic structures, enabling it to distinguish between adjacent elements. In condensed matter chemistry research, scientists have employed neutron diffraction technology to gain deep insights into the structures of complex materials, providing crucial evidence for understanding structure–property relationships.
3.1 Neutron Single-Crystal Diffraction Studies
Single-crystal neutron diffraction (SCND) is primarily used to determine single-crystal structures and plays an important role in the study of coordination compounds, biomacromolecules, magnetic materials, and superconducting materials.
3.1.1 Biomolecules
Due to the high sensitivity of neutrons to light elements, SCND can precisely determine the positions and occupancies of major atoms such as C, H, and O in biomolecules, helping researchers gain a deeper understanding of the crystal structures of proteins, nucleic acids, and other molecules. It reveals important details such as hydrogen-bonding networks, protonation states, and dynamic conformational changes, thereby providing crucial information for elucidating the functional mechanisms of biomolecules.
We can utilize SCND to investigate the planarity of peptide bonds in protein molecules and their related functional mechanisms. The peptide bond is the chemical bond that connects amino acids in proteins, and its planarity is considered the foundation for protein structural stability, influencing hydrogen bond formation and chemical reactions. Traditional theory holds that the six atoms of a peptide bond lie in the same plane; however, high-resolution X-ray diffraction and theoretical calculations have revealed that peptide bonds exhibit distortion. To further explore this issue, Hanazono et al.［2］employed SCND to focus on the planarity of the H i —N i —C i-1O i-1region within iron–sulfur proteins (HiPIP). They determined the crystal structure of oxidized HiPIP at 1.2 Å resolution (1 Å=0.1 nm), finding that the amide proton does not reside entirely within the peptide bond plane but is instead displaced toward the acceptor atom (e.g., oxygen) due to hydrogen bonding (Figure 1). Furthermore, the planarity of the peptide bond is largely dependent on the degree of pyramidalization of the nitrogen atom: the greater the pyramidalization, the more pronounced the distortion of the H—N—CO plane. The data also indicate that four amide protons interact with the sulfur atoms of cysteine residues covalently bound to the Fe atom, and that the N—H bond lengths of these amide protons vary among different subclusters. Notably, the orientation of the amide proton of Cys75differs between the reduced and oxidized states, a change that may be associated with the valence state of the iron–sulfur cluster. This study clarified the existence of peptide bond distortion, elucidated the structural details of HiPIP, and provided a reference for investigating other protein molecules with similar structures.
SCND can also be used to study viral drug targets and their associated mechanisms of action. For example, when the human body is invaded by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), it releases interferons that activate PARP enzymes to perform ADP-ribosylation modifications on proteins as a defense mechanism against the virus. However, the macrodomain (Mac1) of SARS-CoV-2 can remove these modifications, thereby helping the virus evade the host immune response; thus, Mac1 serves as a potential drug target. Correy et al.［3］used SCND to determine the protonation states of certain histidine residues in Mac1 under different pH conditions, both in the apo state (without substrate binding) and when bound to adenosine diphosphate ribose (ADPr) (Figure 2). Based on these findings, they elucidated the hydrogen-bonding network near the active site, demonstrating its critical role in maintaining active-site functionality and substrate recognition. The authors further analyzed the water-molecule network surrounding Mac1’s active site and found that it plays an important role in the binding of non-native ligands (fragments): most fragments bind to the active site by forming bridging interactions with water molecules. This provides new insights for designing Mac1 inhibitors—for instance, by introducing specific functional groups to expand the hydrogen-bonding network or by targeting water molecule displacement to optimize inhibitor design.
3.1.2 Magnetic Materials, Superconducting Materials
SCND possesses unique advantages in the study of magnetic materials. This is because neutrons have a distinct magnetic moment that can interact with the atomic magnetic moments in magnetic materials, allowing for deep penetration down to the atomic scale and precise detection of the orientation of atomic magnetic moments and magnetic structural information within these materials. Furthermore, SCND can directly elucidate spin-ordered states and magnetic phase transition behaviors. Many superconductors exhibit a close interplay between their superconducting state and magnetic order; therefore, researchers can utilize SCND to understand how the orientation of magnetic moments and the magnetic structure change under different conditions. By controlling magnetism in this way, it is possible to optimize the performance of superconductors and provide theoretical guidance for the development of novel superconducting materials.
For example, SCND has been used to study the magnetic properties of FeCrAs. FeCrAs exhibits antiferromagnetic ordering at certain temperatures; however, its DC magnetic susceptibility (χ) shows anomalous behavior as a function of temperature. Jin et al. [4] observed, in SCND experiments, the presence of magnetic reflections with specific propagation vectors in FeCrAs, thereby determining that the onset temperature of its antiferromagnetic ordering is TN = 116(1) K (Fig. 3). Furthermore, they established that the Cr sublattice of FeCrAs displays in-plane helical antiferromagnetic ordering in the ground state. As the temperature increases, the Cr magnetic moments gradually tilt out of the ab plane: the in-plane component decreases, while the out-of-plane component increases. This spin reorientation phenomenon provides a satisfactory explanation for the anomaly observed in the DC magnetic susceptibility around 100 K (T0). These findings reveal the evolution of the magnetic structure of FeCrAs across different temperatures and offer crucial insights into its temperature-sensitive magnetic properties.
For example, SCND has also been used to study the superconducting properties of Eu(Fe0.88Ir0.12)2As2and the magnetic ground state of its Eu2+spins. EuFe2As2belongs to the “122-type” iron-based superconductors and contains two magnetic sublattices; superconductivity can be achieved by doping it with impurity atoms. Jin et al.［5］investigated Ir-doped Eu(Fe1-x Ir x ）2As2. In the SCND experiments, the authors observed that the integrated intensity of the (220) T diffraction peak varied smoothly with temperature, while the peak width exhibited a change similar to that of the (008) T diffraction peak, indicating that no tetragonal-to-orthorhombic structural phase transition occurred in this compound. The spin-density wave order of iron was completely suppressed, allowing superconductivity to fully develop. To determine the magnetic ground state of the Eu2+spins, the authors performed additional measurements on several representative diffraction peaks at 25 K (above the magnetic transition temperature of Eu2+) and at base temperature (well below the magnetic transition temperature). As shown in Figure 4, the intensities of the (110) T and (101) T peaks were relatively weak at 25 K but significantly increased at base temperature, suggesting that as the temperature decreased, the Eu2+spins contributed substantially to ferromagnetism. In contrast, the intensity of the (006) T peak did not exhibit any noticeable change during cooling, indicating that the ferromagnetic component of the Eu2+spins within the ab plane was almost zero or undetectable within the experimental error. Taken together, these results suggest that, within the limits of experimental uncertainty, the Eu2+spins are ferromagnetically aligned along the c axis in their ground state. Refined structural analysis yielded an ordered magnetic moment of 7.0 (1) μB. This finding demonstrates that in Eu(Fe0.88Ir0.12)2As2, superconductivity and the ferromagnetic ground state of Eu2+can coexist, which facilitates a deeper understanding of the intrinsic connections and interaction mechanisms between these two phenomena in iron-based superconducting materials.
3.2 Neutron Powder Diffraction Studies
Neutron powder diffraction (NPD) is primarily used to determine the crystal structures of polycrystalline or powdered samples. NPD is characterized by its simplicity, speed, and broad applicability, making it particularly suitable for studying complex materials that are difficult to obtain in high-quality single-crystal form. It plays a crucial role in investigating structure–property relationships in functional materials for practical applications.
3.2.1 Battery Materials
During the electrochemical reactions of battery materials, light elements such as Li, H, and O are critically important. Their positions, concentrations, and dynamic changes throughout the reaction process directly influence battery performance, including battery capacity, cycle life, and charge–discharge efficiency. Given that neutrons exhibit high sensitivity to light elements and possess strong penetrating power, NPD can penetrate deep into the interior of battery materials to precisely track the evolution of light-element atoms within the crystal structure, thereby facilitating the elucidation of the underlying mechanisms governing battery operation.
For example, we can utilize NPD to investigate the dynamic behavior of Li+ in the cathode material FeCl3 for all-solid-state lithium-ion batteries. FeCl3 possesses a unique layered structure, where the interlayers are filled by a honeycomb arrangement composed of Fe3+ octahedra and vacant octahedra in a 2:1 ratio. Li+ ions are capable of inserting into these vacant octahedral sites. Liu et al. ［6］ conducted NPD experiments on FeCl3 samples with varying lithium contents. The authors found that the crystal structure of FeCl3 undergoes transformations during charge–discharge cycling: upon discharge, an intermediate α phase is first formed; as Li+ ions are inserted, the α phase transforms into a more lithium-rich β phase. During charging, however, the β phase does not revert to the original FeCl3 structure via the reverse pathway of lithium insertion; instead, it undergoes a solid-solution delithiation process followed by the formation of a new γ phase. NPD analysis of the β phase reveals that Li+ predominantly occupies the 4e (octahedral) sites, with a small fraction residing at the 4g (tetrahedral) sites. This structural configuration can be regarded as a cation-deficient spinel structure (Figure 5). These findings suggest that, during the initial discharge process, partial migration of Fe atoms may lead to the formation of a cation-deficient spinel-like structure in FeCl3. Such a structure exhibits high stability for both Li+ insertion and extraction over long-term cycling, thereby enhancing the energy storage capacity and cycle life of lithium-ion batteries.
Additionally, in battery materials, transition metals such as Ni, Co, and Mn often coexist, each exerting varying degrees of influence on material performance from different perspectives. By leveraging the differential scattering characteristics of different elements, NPD can clearly distinguish the evolution of these transition metals during charge–discharge processes. Based on this, researchers can gain a deeper understanding of the specific mechanisms by which transition metals affect battery performance, thereby providing solid theoretical support for optimizing battery performance. For example, the layered double perovskite manganate LnBaMn2O5+δ at the A-site (Ln = Pr, Nd) exhibits mixed ionic and electronic conductivity, redox stability, strong resistance to carbon deposition and sulfur poisoning, yet insufficient catalytic activity. The precipitation of metal nanoparticles from the host oxide lattice represents a promising approach to improving its performance and stability. Bahout et al.［7］conducted an in-situ high-temperature NPD study on (PrBa)1-x/2Mn2-x Ni x O6-δ (x = 0.05), denoted as PBMN. Upon heating the sample under a wet hydrogen atmosphere, new diffraction peaks corresponding to MnO emerged when the temperature approached 400 ℃, accompanied by a decrease in the oxygen occupancy at the O1 site within the body-centered phase (orthorhombic phase) and a concomitant increase in MnO content. At 500 ℃, the orthorhombic phase transformed into a tetragonal phase. By 665 ℃, the tetragonal phase further transitioned into a cubic phase. As the temperature neared 900 ℃, the layered double perovskite PrBaMn2O5phase appeared, while Ni2+precipitated from the structure and was reduced to metallic nanoparticles. During the cooling process (from 900 ℃ to room temperature), the space group of the layered double perovskite changed from P4/mmmto P4/nmm at 550 ℃, attributable to the charge ordering of Mn3+ and Mn2+. Furthermore, analysis of the full width at half maximum of the Ni (111) peak yielded an estimated average crystallite size of 60 nm for the precipitated Ni nanoparticles. These findings elucidate the structural evolution of the material under a H2atmosphere and provide a material-structural basis for understanding how nickel precipitation can optimize the electrochemical performance of PBMN in a H2atmosphere.
3.2.2 Adsorption Materials
NPD is widely used in the study of adsorption materials. Due to its high sensitivity to light elements, NPD can be employed to determine the structure of adsorbates rich in light elements within adsorption materials, thereby elucidating the interaction mechanisms between adsorbates and adsorption materials and providing a basis for interpreting adsorption performance. This contributes to the optimization of adsorption materials and enhances their adsorption efficiency and selectivity.
TAMOF-1 is a novel metal–organic framework material (MOFs) with strong adsorption capacity for CO2. To determine the location of CO2 molecules within the pores of TAMOF-1, Capelo-Avilés et al. [8] employed NPD to resolve the crystal structure of CO2-adsorbed TAMOF-1. The authors found that TAMOF-1 possesses triangular void channels along the [111] direction. After CO2 adsorption, these channels are filled with CO2 molecules exhibiting moderate disorder, indicating weak interactions between CO2 and the TAMOF-1 framework. As shown in Figure 6, some CO2 molecules reside in the narrower channels formed by three Cu atoms, where they exhibit distinct directional interactions with two Cu atoms. The remaining CO2 molecules are randomly distributed throughout the channels. These findings elucidate the adsorption sites and interaction modes of CO2 within TAMOF-1, providing direct evidence for understanding its adsorption mechanism.
Chai et al.［9］ Using NPD, they investigated Ni@FAU zeolite molecular sieves with acetylene/ethylene separation capability. This study not only clarified the precise location of Ni(II) within the FAU framework but also successfully elucidated the adsorption sites and modes of gas molecules. As shown in Figure 7, in the acetylene-adsorbed species ［Ni12Na20（Al44Si148O384）］·（C2D2）12, all adsorbed acetylene molecules reside at a single site within the supercage, interacting with the Ni(II) moiety to form a metastable ［Ni(II)（C2D2）3］ complex, while simultaneously establishing hydrogen bonds with framework oxygen atoms. In contrast, in the ethylene-adsorbed species ［Ni12Na20（Al44Si148O384）］·（C2D4）17, the larger molecular size of ethylene results in only a small fraction of the molecules interacting with Ni(II), with the majority stabilized solely through weak hydrogen bonding with framework oxygen or via intermolecular guest–guest interactions. These findings reveal the highly selective acetylene adsorption mechanism of Ni@FAU, which holds significant implications for guiding the development of novel adsorbents.
3.2.3 Catalytic Materials
NPD can precisely determine the lattice parameters of catalytic materials and the coordination environment of their active sites. It can also track structural changes in catalytic materials under in situ conditions during the reaction process, which helps to gain a deeper understanding of catalytic reaction mechanisms, thereby optimizing catalytic materials and enhancing catalytic efficiency.
Trickett et al.［10］ Utilized NPD to investigate the Brønsted acid active sites of the solid acid catalyst MOF-808-SO4. By means of NPD, the authors ruled out terminal hydroxyl groups and protons on sulfate as the sources of strong acidity. Combined with other experimental evidence, they ultimately determined that terminal water molecules are the source of strong acidity: the strong Brønsted acid active sites are formed through hydrogen bonding between water and chelated sulfate groups. These acid active sites significantly influence the catalytic performance of MOF-808-SO4; the catalytic activity of the material decreases markedly after dehydration. The precise structural and compositional analysis provided by NPD serves as an essential basis for constructing and validating the structural model of the acid active sites, making it an indispensable step.
3.3 Study of the Neutron Pair Distribution Function
The neutron pair distribution function (nPDF) is a radial distribution function between atoms obtained through neutron scattering experiments. Specifically, it statistically represents the distance distribution among all atomic pairs within a material, thereby revealing the local atomic arrangement and providing insights into the material's internal structure. Its mathematical expression can be found in Equation (1)..
G（r） =
in the equation: ρ0represents the average atomic density; S(Q) denotes the structure factor. This function, via Fourier transformation, converts scattering data into a probability distribution of interatomic distances, thereby overcoming the limitation of conventional diffraction techniques that can only resolve long-range ordered structures. It is particularly suitable for analyzing the local structure of amorphous materials, nanostructures, and complex crystals.
3.3.1 Liquid Materials
nPDF can clearly present the atomic distance distribution, local coordination environment, and dynamic changes within liquid materials, thereby enabling researchers to understand the microscopic structure inside liquids. This helps in comprehending the intrinsic nature of liquid material properties and provides a theoretical basis at the microscopic structural level for optimizing processes such as metal smelting and casting.
ZIFs are a class of MOFs that can be melted at high temperatures to obtain a liquid. Gaillac et al.［11］performed RMC modeling on the nPDF data of melt-quenched ZIF-4 and obtained its structural model (Figure 8a). The results show that the melt-quenched ZIF-4 retains the Zn—Im—Zn bonds, tetrahedral coordination features, and the network structure of crystalline materials. Its continuous random structure is similar to that of amorphous SiO2. The authors also conducted synchrotron radiation PDF studies on liquid ZIF-4 and performed RMC modeling of liquid ZIF-4 based on the structural model of the melt-quenched material (Figure 8b). They found that ZIF-4 in the liquid state possesses a unique porous structure. Unlike traditional organic porous liquids, it belongs to an intrinsic porous liquid. This study reveals the distinctive properties of liquid ZIF-4. Such intrinsic porosity in the liquid phase may have significant application potential in areas such as liquid-phase separation and catalysis, providing new directions and insights for expanding the applications of MOFs.
3.3.2 Molten Materials
In the study of molten materials, nPDF can precisely analyze the spatial distribution and dynamic changes of atoms in the molten state, identifying interatomic distances and coordination environments. This information helps to control the melting process parameters, optimize the material forming process, and enhance material quality. It also provides critical data support for an in-depth investigation of the material formation mechanisms.
For example, nPDF can be used to study the coolant salt of molten salt reactors (MSR). As the MSR operates, the composition of the coolant salt changes due to corrosion and radiation, leading to alterations in the key physical properties of the melt. Among these, Cr3+ is the primary corrosion product. Li et al.［12］ employed nPDF to investigate the near-eutectic mixture of NaCl–CrCl3. By combining nPDF with first-principles molecular dynamics simulations, the authors clarified the existence of an octahedral structure of CrCl(Figure 9).
In addition to observing the octahedral structure, the authors also determined the coordination environment of Cr3+ by Cl-, accurately calculated the coordination number, and clearly presented the atomic arrangement and interatomic relationships in the molten salt at the microscopic level. Furthermore, they compared the positions and intensities of characteristic peaks in the nPDF spectra of pure NaCl and NaCl-0 CrCl3, finding that, due to the presence of Cr3+, the Cl—Cl interatomic distance in NaCl-0 CrCl3 is shorter than that in pure NaCl. The effect of Cr3+ on Na+ is relatively minor; consequently, the Na—Na and Na—Cl distances remain nearly identical in both samples. These conclusions help researchers gain a deeper understanding of the influence of Cr3+ on the structure of molten salts, accurately predict their properties, and thereby optimize the performance and safety of molten salt reactors.
3.3.3 Amorphous Materials
Amorphous materials lack long-range periodicity in atomic arrangement, but they exhibit short-range order (such as tetrahedral coordination and layered structures) and intermediate-range order (such as nanoclusters and chain-like arrangements). By using nPDF, it is possible to determine the distribution patterns of atoms within the short-range/intermediate-range ordered regions, such as the average interatomic distance and the number of nearest neighbors, thereby revealing the local atomic arrangement.
NAGP (Na1+x Al x Ge2-x (PO4)3) amorphous materials hold significant application potential in the field of glass-ceramic technology. Understanding their structural characteristics is crucial for optimizing material performance. Gammond et al. [13] conducted nPDF studies on NAGP glassy materials with varying compositions and found that their total structure factor function exhibits a shoulder peak at k1 ≈ 0.85 Å-1. This feature indicates that, within the glassy state, atoms display a certain degree of order at a real-space scale of approximately 2π/k1 ≈ 7.4 Å-1. This observation suggests that the atomic distribution in glassy materials is not entirely random; rather, it exhibits a localized regularity, which is of great significance for comprehending the structural features of glassy materials.
Through fitting analysis of the nPDF data, the authors obtained coordination information for atoms in NAGP glassy materials with different compositions. For the glassy material with x = 0, the P—O coordination number is close to 4, while the Ge—O coordination number is approximately 5. For glassy materials with x > 0 (e.g., x = 0.4 and x = 0.8), the P—O coordination number remains around 4 within experimental error; however, as the aluminum content increases, the average Ge—O coordination number shows a decreasing trend. These results differ significantly from those of crystalline NAGP materials, thereby highlighting the unique atomic coordination environment in the glassy state.
In addition, by comparing the nPDF data of glassy materials in different states (initial state and annealed state) (Table 2), it can be observed that after annealing, the Ge—O coordination number (nGe—O) increases relative to the initial state, whereas the P—O coordination number (np—O (sum)) shows no significant change within the experimental error. This indicates that the annealing process has a pronounced effect on the coordination environment of Ge in the glassy material, driving it toward a structure more similar to that of the crystalline phase, while having only a minor influence on the coordination environment of P. This finding elucidates how annealing alters the atomic coordination environment of the material and provides guidance for selecting appropriate annealing conditions to enhance the material’s structure and properties.
3.3.4 Nanomaterials
nPDF plays a unique role in nanomaterials research. It can probe down to the nanoscale to investigate atomic-level structural details of nanomaterials, revealing atomic distribution and their surrounding coordination environments, as well as structural dynamic changes during reaction processes. This facilitates the elucidation of the properties and action mechanisms of nanomaterials and provides a microscopic structural foundation for designing more efficient nanomaterials.
For example, Pan et al. ［14］prepared long-range ordered porous carbon (LOPC) from C60 under ambient pressure. By comparing the nPDF data of LOPC with those of the pristine C60 molecular crystal and the C60 polymer crystal, they analyzed the structural features of LOPC. As shown in Figure 10, within short-range distances (< 3 Å), the primary types of chemical bonds in LOPC are similar to those in C60. Between 3 and 8 Å (1 Å = 0.1 nm), the atomic spacings in LOPC shift toward longer ranges, indicating that LOPC also possesses a graphite-like structure. At longer ranges (> 9 Å), the degree of disorder in the LOPC structure increases, and the atomic arrangement becomes more irregular. These findings reveal that LOPC exhibits coexisting short-range order and medium-range disorder, which provides a basis for understanding its high electrical conductivity and thermal stability. After conducting comprehensive structural and property characterizations, the authors concluded that LOPC is a metastable intermediate phase during the transformation from fullerene-based to graphene-based carbon structures. It maintains long-range order below 500 ℃; however, as the annealing temperature increases, its properties gradually transition from semiconductor-like to metal-like behavior.
For example, Zhu et al. ［15］conducted an nPDF study on the structure of CeO2nanoparticles. The results indicated that at low temperatures (< 75 ℃), a local structural transformation from the tetragonal phase to the cubic phase occurs in CeO2nanoparticles (Figure 11) ［15］. During this phase transition, O2-ions undergo displacement, leading to a reduction in the unit cell volume. This reveals atomic-scale structural changes in CeO2nanoparticles and provides experimental evidence for theoretical assumptions regarding oxygen vacancies and structural stability, thereby facilitating a deeper understanding of their high oxygen storage capacity and catalytic activity.
3.3.5 Solid Materials
In the field of solid materials, nPDF is an important method for analyzing material structures. It can probe down to the atomic scale to precisely determine relevant parameters, atomic positions, and thermal vibrations, clearly presenting the distribution of interatomic distances and coordination environments within the material. This helps in understanding the intrinsic relationship between material structure and properties, providing critical data support for optimizing the design of solid materials and investigating the mechanisms underlying their structure–property relationships, thereby advancing the research and development and application of novel solid materials.
For example, LiFePO4has become an ideal cathode material for lithium-ion batteries due to its low cost and high theoretical capacity; its crystal structure is similar to that of FePO4after complete delithiation. Sławiński et al.［16］found through nPDF studies that there is no significant local disorder in the lattice of either material. However, a small number of anions in FePO4deviate from their ideal positions. Specifically, approximately 1.7% of O1anions and 2.3% of O2anions exhibit noticeable displacements. This phenomenon is associated with incomplete delithiation of the material and the formation of anti-site (Li+-Fe2+) defects during the delithiation process. These defects may affect the diffusion pathways and rates of Li+ions, thereby influencing the charge–discharge performance of the battery. To address these issues, the delithiation process can be optimized to reduce defect concentrations, thus enhancing the electrochemical performance of the material.
For example, Telford et al.［17］used nPDF to analyze the structure of La1-x Sr x Fe1-y Mn y O3-δ oxygen-storage materials in both oxidized and reduced states. For the oxidized phase, La0.6Sr0.4Fe0.67Mn0.33O3 (LSFM), based on bond-length distribution analysis and valence-sum calculations, the authors found that the valence of Mn is significantly higher than that of Fe. The Mn—O bond lengths exhibit a “bimodal distribution” with a high proportion of short bonds, approaching the characteristic bond length of Mn4+—O. This indicates that Mn predominantly exists as Mn4+in the oxidized LSFM and tends to form well-ordered MnO6 octahedral structures. In contrast, for the reduced-state material, fitting analysis of the nPDF data reveals that oxygen vacancies induce changes in the coordination environments of B-site ions (Fe and Mn), leading to the formation of BO5 tetragonal pyramidal–dominated local defects, which further affect the material’s oxygen-storage performance. These findings elucidate the specific relationship between the local structure and oxygen-storage properties of this material and provide important guidance for designing more efficient oxygen-storage materials—for instance, by adjusting the doping levels of Mn or Sr to enhance oxygen storage capacity.
4 Neutron Spectroscopy Studies in Condensed Matter Chemistry
4.1 Inelastic Neutron Scattering Studies
Inelastic neutron scattering (INS) is a neutron energy spectrum technique that monitors the breaking or formation of chemical bonds and molecular conformation by detecting changes in neutron energy after interaction with a sample. The following are some examples of INS applications.
4.1.1 Catalytic Materials
In multiphase catalysis research, elucidating the surface species states of catalysts is crucial for understanding reaction mechanisms. Infrared spectroscopy and Raman spectroscopy have been widely employed for this purpose. However, due to optical selection rules and complex modeling challenges, these techniques cannot precisely characterize the surface species states of catalysts. INS, based on the principle of neutron-nucleus scattering, effectively circumvents optical selection rules and involves relatively simple modeling. Consequently, it can clearly reveal the surface species states of catalysts and serves as an important experimental tool in catalysis research［18］.
For example, we can utilize INS to investigate the influence of organic shell layers on catalyst surfaces on catalytic efficiency. Nunes et al. ［19］employed INS to study how organic shell layers on the surface of MoO2nanocatalysts affect the catalytic efficiency of the styrene oxidation reaction. The authors examined both the HQMoO2catalyst synthesized using ethylenediamine/hydroquinone and the FCMoO2catalyst synthesized using ethylenediamine/Fe2O3. By comparing the INS spectra before and after the reaction, they found that the HQMoO2surface exhibited strong adsorption toward the ethylenediamine/hydroquinone shell layer, which suppressed the reaction activity and resulted in only trace amounts of product formation. In contrast, the FCMoO2surface displayed only weak interactions with the organic shell layer; the loosely adsorbed shell layer was readily displaced during the reaction, thereby facilitating the formation of oxidative products.
For example, we can employ INS to investigate the interactions between reaction substrates and catalytic active sites, thereby deducing the reaction mechanism. Ma et al.［20］used INS to study the catalytic methane conversion mechanism within MOFs. Directly converting methane into ethylene and acetylene represents an important pathway for the high-value utilization of natural gas. The authors designed MFM-300(Fe) to achieve efficient catalytic methane conversion. Subsequently, INS was employed to examine the state of methane within MFM-300(Fe) (Figure 12). Upon methane loading, a characteristic peak at 55–60 meV associated with O—H rocking vibrations exhibited a redshift, indicating host–guest interactions between methane molecules and the Fe—O(H)—Fe sites. Notably, this feature persisted even under vacuum conditions, further confirming the strong binding of methane molecules to the MFM-300(Fe) framework. Combined with theoretical calculations, the INS data revealed that the interaction between methane molecules and the Fe—O(H)—Fe sites effectively lowered the activation energy barrier for C—H bond cleavage. These findings elucidated the critical roles of the Fe—O(H)—Fe sites in adsorbing methane molecules, reducing the C—H bond activation energy, and stabilizing reaction intermediates during methane conversion to ethylene and acetylene, thereby providing theoretical support for the design of highly efficient catalysts.
4.1.2 Magnetic Materials
Single-molecule magnets (SMMs) hold potential applications in fields such as quantum information. Lanthanide complexes, owing to the strong spin–orbit coupling and magnetic anisotropy of their 4f electrons, have emerged as key materials for high-performance SMMs. However, electron paramagnetic resonance spectroscopy (EPR) and far-infrared spectroscopy (FIR), due to their limited sensitivity toward lanthanide elements, are unable to accurately elucidate the mechanisms involving 4f electrons. Inelastic neutron scattering (INS), with its unique magnetic sensitivity, can be employed to investigate the interactions between lanthanide ions and ligands. Dunstan et al.［21］utilized INS to study the mechanism of magnetic exchange coupling between lanthanides and free radicals. The authors examined the Er complex containing a paramagnetic semiquinone ligand, Er-dbsq, as well as its partially deuterated counterpart, Er-dbsqD. By comparing the INS spectra of Er-dbsqD and Er-dbsq, they found that the Q-dependence of the 0.918 meV feature peak did not conform to either the rapid-decay model of pure free-radical spin flips or the slow-decay model of pure 4f transitions. Combining these findings with theoretical calculations, they concluded that this characteristic peak arises from nonlocal hybridization between the 4f electrons of Er3+ and the π orbitals of the semiquinone ligand, leading to quantum interference in the magnetic form factor between the contributions of Er and the free radical. This suggests the presence of antiferromagnetic exchange interaction between Er and the free radical. Such conclusions can help researchers gain deeper insights into and better understand the intricate magnetic coupling systems in magnetic materials.
INS can also be used to study the key magnetic characteristics of magnetic materials and their atomic vibrational patterns. Xie et al. ［22］employed INS technology to investigate the magneto-dynamic behavior of the high-temperature superconducting bilayer nickelate La₃Ni₂O₇₋ δ. By comparing the polycrystalline INS data of La₃Ni₂O₇₋ δ collected at 10 K and 160 K, the experiment revealed that the material exhibits no long-range magnetic order even down to 10 K under ambient pressure, suggesting that this phenomenon arises from the extremely small magnetic moments of Ni atoms, which make the magnetic signal difficult to detect. Subsequently, by subtracting the high-temperature data from the low-temperature data to eliminate lattice vibrational interference, a spin-excitation signal of magnetic origin was identified at an energy of 45 meV. The variation in the intensity of this signal is consistent with the magnetic properties of Ni2+ ions. Finally, combined with theoretical simulations, it was confirmed that the material possesses the characteristic feature of “strong interlayer magnetic coupling and weak intralayer magnetic coupling” (interlayer coupling energy ranging from 57.5 to 63.9 meV, whereas intralayer coupling energy is only 3.0 to 3.6 meV). Throughout this process, INS not only addressed the qualitative question of whether magnetic order exists but also enabled in-depth analyses such as the localization of spin excitations and the quantitative determination of magnetic coupling, thereby providing crucial data for elucidating its superconducting mechanism.
4.1.3 Hydrogen storage materials
INS can provide unique insights into the interactions between H2 molecules and hydrogen storage materials. For example, Ciocarlan et al.［23］used INS to reveal the interactions associated with H2 molecules within the nanopores of mesoporous silica. In this experiment, hydrophobically modified phenethyl-functionalized mesoporous silica (MCFPhene) was selected as the research object, and the adsorption, storage, and release processes of H2 therein were monitored in real time. As shown in Figure 13, a characteristic peak at 12.3 meV appeared under 135 MPa, corresponding to the J = 0→1 rotational transition of H2 molecules, which demonstrated that H2 molecules interacted with the silica surface via surface hydrogen bonding and were adsorbed onto the hydrophobic pore walls. When the pressure increased to 165 MPa, a characteristic peak at 14.6 meV emerged, indicating the formation of sII-type H2 hydrate, thereby proving that H2 was successfully encapsulated within the hydrate cages, realizing a dual hydrogen storage mode of "hydrate cage + physical adsorption."
4.2 Quasi-elastic neutron scattering studies
Quasielastic neutron scattering (QENS) is capable of measuring dynamical processes on picosecond to nanosecond timescales and angstrom to nanometer length scales. It can be used to study molecular/ion diffusion/transport processes as well as conformational/segmental dynamics of macromolecules［24］. Several simple examples are provided below for illustration.
4.2.1 Energy Materials
QENS can be used to study the diffusion behavior of substrate molecules in catalytic materials. For example, Davidson et al. ［25］employed QENS to investigate the diffusion behavior of H2molecules in iron-based Fischer–Tropsch catalysts. Fischer–Tropsch synthesis is an important industrial catalytic reaction that converts syngas into hydrocarbons. In the initial stage of the reaction, iron-based catalysts dynamically evolve into multiple phases, which affect the diffusion behavior of H2,thereby determining the reaction efficiency. To address this issue, the authors studied the diffusion characteristics of H2in iron-based catalysts pre-treated for different durations. They first pre-treated the iron-based catalysts under reaction conditions for 0, 12, and 24 hours (FT0, FT12, and FT24). The diffusion rate of H2molecules in the FT0 sample did not vary with temperature (Figure 14a). However, the diffusion rates of H2molecules in the FT12 and FT24 samples exhibited significant temperature dependence (Figure 14b, c). Moreover, the longer the pretreatment time, the more pronounced the temperature dependence of the diffusion rate. This indicates that a pretreatment duration of 12 to 24 hours indeed induces changes in the catalyst structure, thereby influencing the diffusion behavior of H2molecules. These findings provide new evidence for optimizing the Fischer–Tropsch catalytic process.
QENS can also be used to study the ion transport mechanisms in batteries. Willis et al. ［26］employed QENS to investigate the diffusion mechanism of Na+in sodium-ion batteries. Due to its abundant resources and low cost, Na+has gradually become a research hotspot in the energy storage field. A deep understanding of the diffusion mechanism of Na+within battery materials is of great significance for optimizing battery performance. The authors focused on layered sodium cobalt oxide (Na x CoO2) materials and utilized QENS to monitor the dynamic behavior of Na+, thereby elucidating its transport mechanism. They selected Na x CoO2samples with varying sodium contents as the research objects and employed QENS to confirm that the transport pathway of Na+involves a 2d-to-2bsite hopping process. Furthermore, they determined the dimensional characteristics of Na+transport: namely, one-dimensional diffusion occurs within partially disordered stripes, whereas it transitions to two-dimensional superionic diffusion in the fully disordered phase. QENS also provided the activation energies for Na+transport at different temperatures: in the fully disordered phase at high temperatures, the activation energy is lower, at 85 meV, resulting in faster diffusion; whereas in the partially disordered stripe phase at intermediate temperatures, the activation energy is higher, at 170 meV, leading to slower diffusion. The experimental results indicate that the transport rate of Na+is governed by its arrangement within the crystal lattice—ordered structures inhibit diffusion, while disordered structures promote it.
QENS can also be used to study the proton transport mechanism in proton-conducting materials. Proton-conducting materials play an important role in energy conversion and storage devices such as fuel cells. Pili et al. ［27］synthesized a novel phosphorus-based MOF proton-conducting material, MFM-500 (Ni), and employed QENS to investigate its proton conduction mechanism in depth. The elastic incoherent structure factor (EISF) curves extracted from the QENS data exhibited a clear Qdependence (Figure 15a). Fitting revealed that proton diffusion in MFM-500 (Ni) best conforms to the “intra-spherical free diffusion” model (Figure 15b), rather than the conventional “site-to-site hopping” model. Protons diffuse freely within a spherical region of 2.25 Å in radius, a distance consistent with the hydrogen-bonding distances observed in the crystal structure. When the diffusion distance exceeds 2.25 Å, proton transport requires the assistance of water molecules, exhibiting a mixed characteristic of both the Grotthuss (proton hopping) and Vehicle (carrier-mediated) mechanisms, with an activation energy lying between those of the two mechanisms. This work represents the first observation of the “intra-spherical free diffusion” proton conduction mechanism in MOFs, providing a theoretical basis for the design of novel proton-conducting materials.
4.2.2 Biomolecules
QENS plays a unique role in the study of biomolecules, primarily used to probe the conformational dynamics of macromolecules. For example, Shrestha et al.［28］employed QENS to elucidate the relationship between the conformational dynamics of G-protein-coupled receptor (GPCR) rhodopsin and its cellular signaling function. The authors conducted QENS measurements on dark-state rhodopsin (bound to 11-cis retinal ligand) and opsin (ligand-free) within the temperature range of 220–300 K. As shown in Figure 16,the hydrogen atom relaxation time τof dark-state rhodopsin is shorter, indicating faster motion, which suggests that the ligand locks the protein into a specific conformation. In contrast, the τof opsin is significantly prolonged, with restricted hydrogen atom motion and higher activation energy, implying that the opsin conformation formed after ligand dissociation is more open. This indicates that the ligand suppresses protein activity by restricting conformational freedom, whereas upon ligand dissociation, increased conformational freedom facilitates binding with signaling molecules. These findings highlight the key mechanism underlying GPCR activation: ligand dissociation induces conformational opening and dynamic changes.
4.2.3 Polymer Materials
The application of QENS in polymer materials is primarily used to study their segmental dynamics. For example, Kim et al.［29］employed QENS to investigate the relationship between the segmental dynamics of complex coacervates and their composition as well as environmental factors. The authors prepared a series of complex coacervates by mixing polyelectrolyte solutions containing ammonium (A), guanidinium (G), or sulfonic acid (S) groups. The QENS results showed that, as the salt concentration increased from 0 to 1.8 mol/L, the local diffusion coefficient of the A + S complex coacervate rose from 62.4 μm2/s to 107 μm2/s (Figure 17a). This indicates that salt can shield electrostatic interactions, reducing ion pairs between ammonium and sulfonic acid groups, thereby increasing the number of mobile segments and accelerating their motion. In contrast, the diffusion coefficient of the guanidinium-based complex coacervate (G + S) remained nearly unchanged with varying salt concentrations (Figure 17b), and its overall diffusion was significantly slower than that of the ammonium-based system. This suggests that stronger non-electrostatic interactions exist between guanidinium ions and sulfonate groups, forming more stable ion pairs that are less susceptible to disruption by salt. Additionally, the guanidinium system exhibits lower water content and lacks sufficient water molecules for lubrication, resulting in segments that are “tightly constrained” and difficult to move. Furthermore, the experiments revealed that the diffusion coefficients of the A + S system were similar at high temperature (300 K, 0 mol/L salt) and low temperature (286 K, 1.2 mol/L salt), indicating that temperature and salt concentration can equivalently influence the segmental dynamics of ammonium-based complex coacervates by modulating the strength of electrostatic interactions. On the other hand, the segmental dynamics of guanidinium-based complex coacervates are predominantly governed by non-electrostatic interactions. QENS provides a quantitative explanation of the nanoscale segmental dynamics within coacervates, demonstrating that the dynamics of polyelectrolyte segments are jointly determined by ion bond strength, the chemical structure of ionic groups, salt concentration, and water content.
5 Neutron Imaging/Scattering Studies in Condensed Matter Chemistry
5.1 Neutron Imaging Research
When a neutron beam is incident upon an object, it interacts with the object through processes such as scattering and absorption. The stronger the scattering or absorption of neutrons by the sample, the lower the intensity of neutrons that penetrate the sample will be, resulting in darker image colors (lower grayscale values) on the detector. By analyzing the varying degrees of darkness in the image—that is, by measuring the distribution of neutron intensity transmitted through the object—it is possible to obtain information about the spatial distribution of different elements within the object. Common neutron imaging techniques (Neutron imaging, NI) include neutron radiography (Neutron radiography, NR) and neutron tomography (Neutron tomography, NT). NR is a technique in which a two-dimensional projection image of the object is formed on a detector after the neutron beam has passed through the object. NR can intuitively reveal the general internal structure and defects of the object. In contrast, NT involves rotating the sample in the neutron beam and acquiring exit beams at multiple angles following their interaction with the object; these data are then processed using mathematical algorithms to reconstruct three-dimensional structural information of the object's interior, thereby enabling comprehensive and precise observation of its internal architecture. NT can be further subdivided into neutron computed tomography (Neutron computed tomography, NCT), neutron diffraction tomography, and other methods. Neutrons possess strong penetrating power; even when the object is relatively thick or contains materials opaque to X-rays, neutrons can still pass through while carrying internal information. This characteristic confers a unique advantage to neutron imaging techniques in nondestructive testing. Below, several specific examples are presented to illustrate the applications of neutron imaging techniques in condensed matter chemistry research.
5.1.1 Electrochemical Studies
Neutron imaging technology can provide real-time, intuitive visualization of the internal structural evolution within batteries, which is conducive to optimizing battery design and enhancing their performance. For example, leveraging neutron imaging's high sensitivity to lithium, Sun et al. ［30］conducted ex situ NCT on lithium–oxygen batteries at different cycle numbers, in conjunction with X-ray computed tomography, and observed that the morphology of the lithium anode underwent significant changes after cycling (from a blocky structure to a porous structure). As the number of cycles increased, the extent of this morphological transformation became more pronounced and irreversible; specifically, the lithium on the anode could not revert to its original blocky configuration during charging. The greater the degree of morphological transformation of the lithium anode, the more severe the decline in electrochemical performance; once the anode morphology had almost completely transformed, the battery would fail. This conclusion vividly demonstrates the impact of lithium anode structural changes on battery performance and provides crucial theoretical support for designing lithium–oxygen batteries with higher performance and longer service life.
Neutron imaging technology can also provide real-time, intuitive visualization of structural evolution within the electrolyzer, which helps to optimize its design and enhance performance. For example, Disch et al. ［31］used neutron imaging to investigate the effects of salt precipitation, water transport, and local current density distribution on the electrolysis process in a zero-gap CO2electrolyzer. In the NR images, precipitated salts within the cathode gas diffusion electrode (GDE) can be clearly observed (Figure 18). These salts appear as dark streaks that penetrate the GDE, with greater accumulation in the channel regions compared to the land regions. Salt precipitation severely hinders CO2transportation and is a major cause of reduced Faraday efficiency. Additionally, dynamic changes in water distribution on both the cathode and anode sides can be observed under different current densities. On the cathode side, as the current density increases, water consumption and electroosmotic drag progressively dry out the cathode GDE; however, in certain areas—for instance, at 25 mA/cm2—the GDE remains moist due to hydroxide ions dragging water into the electrode. Conversely, on the anode side, oxygen evolution leads to increasing bubble accumulation as the current density rises. Such dynamic variations in water distribution also affect battery efficiency. The study further revealed that the local reaction rate in the cathode land region is relatively high, suggesting that, within a certain current density range (below 300 mA/cm2), the overall cathodic reaction is not limited by gas transport but shifts toward hydrogen evolution when CO2is depleted. Meanwhile, the elevated local reaction rate may induce localized drying and overheating, resulting in ionomer degradation and electrolyzer failure. These findings lay a foundation for in-depth research into CO2electrolysis technology, thereby facilitating optimization of electrolyzer design and improvement of its stability and reaction efficiency.
5.1.2 Catalysis Research
Leveraging its sensitivity to light elements, neutron imaging technology can precisely reveal the distribution of light elements within catalysts, thereby helping to identify active sites. Furthermore, neutron imaging can be used to observe reaction processes in real time, such as tracking the adsorption, diffusion, and transformation of hydrogen-containing reactants within the catalyst. This provides intuitive evidence for understanding catalytic reaction mechanisms and differences in reaction rates among various sites, ultimately enabling optimized catalyst design and enhanced catalytic efficiency.
For example, Cavaye et al. ［32］employed neutron imaging technology to investigate the dynamic evolution of H2 during the hydrogenation process over a palladium–carbon catalyst (Pd/C). The authors found that when H2 was introduced into the Pd/C catalyst bed, there were significant differences in H2 adsorption among various regions within the bed. As shown in Figure 19, the region near the gas inlet (blue area) exhibited an extremely rapid H2 adsorption rate, reaching saturation almost instantaneously; in contrast, the region at the top of the catalyst bed (green area) experienced a much slower adsorption process, requiring approximately 1800 s (30 min) to attain saturation. This finding suggests that gas flow within the catalyst bed encounters resistance, and variations in packing density across different regions can influence gas diffusion and adsorption.
To track the adsorption/absorption process in real time, the authors introduced D2into a catalyst that had reached H2adsorption saturation. They observed that both the green and blue regions within the catalyst bed gradually became lighter (with increasing grayscale values), eventually returning to their initial levels at the start of the experiment. This phenomenon indicates that most of the strongly neutron-signaled H2that had been adsorbed was exchanged with weakly neutron-signaled D2, while the H2/D2exchange rate was significantly lower than both the H2adsorption rate and the rate at which H2reached equilibrium throughout the entire catalyst bed. The authors also identified the initiation site and diffusion process of H2adsorption: H2adsorption commenced from the lower-left corner of the catalyst bed, and over time, the gas adsorption front continuously advanced until the entire sample reached saturation. Notably, gas adsorption occurred more rapidly in cracked regions where the gas had easier access to the catalyst. This finding directly reveals the influence mechanism of the internal structure of the catalyst on gas adsorption, providing direct evidence for the study of chemical reaction kinetics on catalyst surfaces and facilitating a deeper understanding of the properties and reaction mechanisms of active sites on catalyst surfaces.
5.2 Small-Angle Neutron Scattering Studies
Small angle neutron scattering (SANS) is an important technique for probing the mesoscopic and macroscopic structures of materials. It can provide structural information at the nanometer to micrometer scale, such as particle size, shape, distribution, and intermolecular interactions. In SANS, Qis a crucial concept. Qrepresents the scattering vector, which is related to the incident wavevector and the scattered wavevector of neutrons. Its magnitude is given by Q =sin（），where λdenotes the wavelength of the incident neutrons, and θis the scattering angle. Different Qregions correspond to structural information at different length scales. The low-Qregion (0.007–0.06 Å-1) reflects the macroscopic structural features of the material; the mid-Qregion (0.06–0.15 Å-1) provides insights into the specific surface area and solid–liquid interface characteristics; whereas the high-Qregion (> 0.35 Å-1) reveals the internal ordered structure of the material, such as the lattice spacing of crystals or the center-to-center distance of pores. SANS has a wide range of applications and plays a significant role in many cutting-edge research fields within condensed matter chemistry.
5.2.1 Biomedical Research
SANS can precisely determine the structure of biomacromolecules (such as proteins and nucleic acids), including their size, shape, and internal group distribution. It also studies conformational changes in solution environments and intermolecular interactions, thereby identifying interaction sites and strengths. This is of great significance for elucidating the molecular mechanisms underlying life processes and provides crucial evidence for biomedical research.
Hughes et al.［33］ used SANS to investigate the network structures of multimeric proteins with varying aspect ratios. The results showed (Figure 20a) that, at a protein concentration of 25 mg/mL, the fractal dimension (Df) decreased linearly with increasing aspect ratio (AR) of the building blocks; whereas at 50 mg/mL, Df exhibited an S-shaped decreasing trend. The decrease in Df indicates a reduced spatial filling capacity of the network and a more open structure. As shown in Figure 20b, with the increase of AR, the L-protein hydrogel network underwent significant structural changes: at 50 mg/mL, the initially cross-linked fractal clusters gradually transformed into a uniform network of cross-linked multimeric protein rods. This clearly demonstrates the systematic evolution of protein hydrogel network structures as a function of AR, providing direct evidence for understanding the assembly mechanisms of protein networks and facilitating a deeper comprehension of the formation processes of analogous fibrous networks in biological systems. The authors also identified a critical aspect ratio for the transition in network structure. When the AR exceeds this critical value, the dominant mechanism shifts from translational diffusion to rotational diffusion, thereby altering the network architecture. This finding offers key parameters for the precise control of protein network structures, enabling the design of materials with tailored network architectures and desired properties by modulating the AR.
5.2.2 Polymer Materials Research
In polymer materials research, SANS can be used to determine the size, shape, and aggregate structure of polymers, revealing the distribution of different polymer phases within a polymer blend. Under varying temperatures and solvent conditions, polymer chains adopt distinct conformations; SANS is capable of capturing these conformational changes, thereby elucidating the flexibility and rigidity of the molecular chains as well as intermolecular interactions. Furthermore, SANS can be employed to monitor in real time structural changes during phase transitions such as glass transition and crystallization in polymer materials, clarifying the various factors that influence material structure and performance throughout these processes and providing a reference for optimizing polymer material design.
Porous liquids (PLs), as a novel class of porous materials, hold potential applications in industrial processes such as gas adsorption and separation. In PLs constructed from molecular nanocages, the surface layer significantly influences both the gas adsorption properties and the fluidity of the material. Lai et al. ［34］ conducted small-angle neutron scattering (SANS) experiments on PPG-CNC, confirming its core–shell structure and elucidating the structural relationship between the nanocages and the grafted polymers within the PLs—that is, the nanocages serve as the core while the polymers constitute the shell. Furthermore, detailed information regarding the core–shell structure was obtained, including parameters such as the dimensions of each component and their scattering length densities, thereby providing an important structural basis for investigating the fluidity and gas adsorption performance of PLs. The authors also utilized SANS to analyze the properties of the surface polymer layer. The results indicated a relatively high solvent content within the shell layer, suggesting that solvent molecules can penetrate into this surface polymer layer. This degree of penetration affects the extent to which gas molecules come into contact with the internal pores of the nanocages, consequently influencing the efficiency of gas adsorption and diffusion. Specifically, a higher solvent content may impede the access of gas molecules to the interior of the nanocages, leading to a reduction in the amount of gas adsorbed; conversely, a lower solvent content may facilitate closer interaction between gas molecules and the adsorption sites inside the nanocages, thereby enhancing gas adsorption efficiency. These findings provide crucial insights into the mechanisms underlying gas adsorption and diffusion within PLs.
Soft and compressible colloids are an important component of soft matter systems. Understanding the bulk modulus (κ, which measures a material’s resistance to volume change) of their particles is crucial for studying colloid properties. Existing methods for measuring bulk modulus have limitations when applied to nanoscale colloids, necessitating new research approaches. Small-angle neutron scattering (SANS) can provide sample information at the nanoscale and thus holds potential for addressing these challenges. Houston et al.［35］, using poly(N-isopropylacrylamide) (pNIPAM)-based nanogels as the subject, employed partially deuterated polyethylene glycol (d83%PEG) as the osmotic pressure-inducing polymer. By establishing a mathematical model to fit SANS data, they accurately measured the bulk modulus of the nanogels (Figure 21a). The study yielded the following conclusions: First, the higher the degree of cross-linking in the nanogel, the greater its bulk modulus—that is, the stronger its resistance to compression and the harder it becomes. Second, as the osmotic pressure increases and the degree of compression intensifies, the bulk modulus also rises (Figure 21b), indicating that the nanogel gradually hardens under compression. This trend is associated with changes in the internal structure of the nanogel. When compressed, solvent is expelled, leading to an increase in pNIPAM density and consequently to enhanced hardness. Third, the bulk modulus of the nanogel shell is significantly lower than that of the entire particle, and its response to compression differs from that of the core. For example, the shells of highly uncrosslinked nanogels may suddenly collapse under high osmotic pressure, whereas those of more highly crosslinked nanogels exhibit a more regular shrinkage pattern. These findings elucidate the structural transformation mechanisms of nanogels under external forces, which can aid in assessing their stability and functionality in complex physiological environments. Moreover, they lay a solid foundation for investigating the properties of other soft colloidal systems.
5.2.3 Electrochemical Studies
In electrochemical research, SANS can be used to probe the structure of electrode materials, monitor in real time the dynamic behavior of ions during battery charge and discharge, and reveal the formation mechanism of solid-state electrolyte interfaces. These insights provide valuable information for a deeper understanding of electrochemical reaction mechanisms, thereby advancing the development of novel, high-performance electrochemical materials.
For an in-depth investigation of the mechanism by which sulfur is converted into lithium sulfide to address issues such as rapid capacity decay in lithium–sulfur batteries, Prehal et al. ［36］employed in situ SANS to track in real time the structural evolution of lithium–sulfur batteries across length scales from atomic to submicrometer during charge and discharge cycles, arriving at the following conclusions. (1) The structure and distribution of key discharge products were clarified. During discharge, the solid discharge products consist of nanocrystalline Li2S and Li2S2 particles, with Li2S2 particles distributed nearly randomly within the voids formed by Li2S particles, albeit with a slight tendency to aggregate near the Li2S crystallites. The porous architecture created by Li2S and Li2S2 serves as a critical bottleneck for mass transport, thereby limiting battery performance. (2) The dynamic structural changes of these two components during charge and discharge were revealed: during discharge, the size of Li2S aggregates gradually increases, whereas the size of Li2S2 particles initially decreases slightly before beginning to grow again in the early stages of charging; upon charging, the Li2S aggregates dissolve, while the Li2S2 particles first increase in size, with their dissolution lagging behind that of Li2S. (3) These findings provide support for the formation mechanism of Li2S: Li2S is formed through the precipitation of Li2S2 followed by solid-state transformation. Specifically, solution-mediated disproportionation of Li2S4 yields Li2S2, which precipitates, after which Li2S2 undergoes solid-state electroreduction to produce Li2S. These insights are of significant importance for understanding the charge–discharge mechanisms and reaction pathways of lithium–sulfur batteries, providing a basis for explaining practical problems such as capacity fade and charge–discharge efficiency, and facilitating targeted improvements in battery performance.
Using contrast variation small-angle neutron scattering (CV-SANS) and contrast variation ultra-small-angle neutron scattering (CV-USANS) techniques, Balu et al. ［37］investigated the structure of catalyst inks used in proton exchange membrane fuel cells. A proton exchange membrane fuel cell is an electrochemical device capable of converting hydrogen chemical energy into electrical energy. Its core component is the membrane electrode assembly, where the electrode catalyst layer (ECL) is prepared from colloidal catalyst ink. The structure, stability, and aggregate size distribution of the catalyst ink are influenced by the quality of the dispersing medium (DM), which in turn affects the performance of the ECL. The results indicate that the structural changes in the catalyst ink are closely related to the composition of the DM. As the content of isopropanol (IPA) in the DM decreases, the aggregate size of the carbon support diminishes, the thickness of the Nafion ionomer shell becomes thinner, while the radius of Nafion ionomer nanoparticles and the interparticle distance increase. These structural changes arise from alterations in the dielectric constant of the DM, which affect the interactions among the various components. Such findings elucidate the intrinsic mechanisms by which the DM influences the structure of the catalyst ink and provide detailed data for a comprehensive understanding of this structure. By adjusting the composition of the DM, it is possible to control the structure of the catalyst ink, thereby enabling the design of catalyst inks tailored to specific requirements and ultimately enhancing the performance of the ECL.
5.2.4 Catalysis Research
SANS possesses unique capabilities in determining the size and shape distribution of active metals in supported catalysts, the formation and location of coke, as well as the adsorption and diffusion of ions and molecules within porous structures. Furthermore, SANS can also be employed to study polymer-composite catalysts. For example, Koizumi et al. ［38］investigated the catalysts used in polymer electrolyte fuel cells (PEFCs). By combining SANS with contrast variation experiments, the authors determined the thickness and water content of the ionomer thin layer in the PEFC catalysts. The study revealed that catalysts containing an ionomer exhibit hydrophobic characteristics, which lead to the formation of voids around the carbon particles, resulting in relatively small O2diffusion losses. In contrast, catalyst powders devoid of an ionomer are completely covered by water, causing substantial O2diffusion losses and thereby reducing performance. Adjusting the content or structure of the ionomer can optimize catalyst performance, thereby enhancing fuel cell efficiency. Such insights facilitate further molecular-level optimization of fuel cell catalysts.
6 Challenges and Opportunities
6.1 Technical Challenges
Neutron scattering techniques still face multiple challenges in practical applications. First, due to the significantly lower flux of neutron sources compared with synchrotron radiation sources, neutron scattering experiments generally suffer from a time-consuming bottleneck, which limits their capability for high-time-resolution observation of dynamic processes. Currently, efforts are mainly focused on enhancing neutron flux by relying on high-flux reactors (such as ILL) or pulsed spallation neutron sources (such as SNS and J-PARC). The next-generation neutron sources under construction (e.g., ESS), through optimized accelerator and target station designs, are expected to increase neutron flux by two to three orders of magnitude, potentially overcoming the "slow-experiment" constraint. Second, neutron scattering data are characterized by strong background noise and complex multidimensional features. Therefore, it is necessary to combine Monte Carlo simulations to precisely correct the instrument response function and to introduce machine learning algorithms for data dimensionality reduction and feature extraction, thereby improving the accuracy of data analysis. Finally, there remains a methodological gap in constructing multimodal correlation analysis systems. Specifically, how to achieve spatiotemporal scale matching and physical model coupling between the unique nuclear and magnetic sensitivity of neutron scattering, the electronic structure information obtained from synchrotron X-ray experiments, and the microscopic mechanisms inferred from theoretical calculations urgently requires the development of cross-modal data fusion technologies and standardized analytical frameworks. Breakthroughs in these core issues will profoundly influence the depth of application of neutron scattering techniques in cutting-edge research fields of condensed matter chemistry.
6.2 Directions for Development
The future development of neutron scattering technology will focus on three major directions. First, the next-generation large-scale scientific facilities, exemplified by the European Spallation Source (ESS) and the second phase of the Spallation Neutron Source (SNS) in the United States, are advancing time resolution to the microsecond level through groundbreaking improvements in neutron source flux and detector technologies. This enables atomic-level dynamic observation of ultrafast dynamical processes, such as battery charge-discharge cycles and catalytic reaction pathways. Second, in situ/operando characterization techniques are being upgraded toward high-throughput and multi-field coupling capabilities. By integrating advanced sample environments, including high-temperature/high-pressure reaction chambers and synchrotron radiation–coupled systems, these advancements are constructing full-scale analytical platforms that span from laboratory simulations to industrial operating conditions, thereby elucidating structure–property relationships under realistic service conditions. Finally, the deep integration of artificial intelligence with multiphysics modeling techniques is driving the development of intelligent data analysis frameworks endowed with autonomous learning capabilities. By fusing multi-modal data from neutron scattering, synchrotron radiation, electron microscopy, spectroscopy, and other modalities, these frameworks are enabling the construction of dynamic digital twin models that span the micro-, meso-, and macro-scales, thus revolutionizing the research paradigm for understanding complex systems.
7 Conclusion and Outlook
Neutron scattering techniques, owing to their unique advantages, are becoming indispensable tools in condensed matter chemistry research. With technological advancements and deepening interdisciplinary collaborations, they will play an even greater role in elucidating the dynamic behaviors of complex chemical systems and guiding the rational design of functional materials. In the future, it will be necessary to further overcome technical bottlenecks and promote a transformative development of neutron scattering technology—from “static analysis” to “dynamic tracking.”
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