Transition metal oxides have strong electron correlation effect due to the existence of partially filled local d or f orbitals in the system^[1]. Surfaces, interfaces and heterojunctions of transition metal oxides usually exhibit rich and complex physical States through the complex coupling of electronic correlation with physical degrees of freedom such as crystal symmetry, electron spin and spin-orbit coupling effect.Including: interface two-dimensional electron gas, interface superconducting state, multiferroics, and exotic magnetic structures (such as nonlinear topological magnetic structures such as the Sgemington state)^{[2][3][4][5][6]}. The theoretical prediction and experimental observation of these novel quantum States provide a rich material system for the future research and development of microscale high-performance oxide electronic devices. However, the growth and synthesis of oxide low-dimensional thin film systems usually require the growth of a certain thickness of the target material on the substrate material by pulsed laser deposition, molecular beam epitaxy, chemical vapor deposition and other methods. Although these methods have achieved great success in the past decades, the existence of substrate materials imposes two limitations on the preparation of target material systems: first, the principle of growing target materials on substrate materials is to form stable chemical bonds between target materials.To some extent, this will bring additional phenomena such as interfacial atom exchange, charge transfer, interfacial reconstruction and orbital hybridization, which will affect the physical and chemical properties of the system when preparing ultra-thin films. Secondly, the substrate material usually has a specific lattice constant, which is usually not equal to the lattice constant of the target material, resulting in additional stress effects, which to some extent affect the electronic and magnetic structure of the target material. Therefore, it is necessary to find a general and efficient method to prepare free-standing oxide thin films.

In 2016, Professor H. Hwang's research team at Stanford University synthesized a strontium aluminate oxide with the chemical formula of H. Hwang], which has a cubic phase structure and a lattice constant similar to that of many transition metal oxides with perovskite structure, so it can be used to prepare stable heterojunction systems^[7]. At the same time, the material has a large number of Sr-O bonds and Al-O bonds which can easily generate electrostatic interaction with water molecules, and has good water solubility, so that the Sr₃Al₂O₆ layer in the heterojunction can be dissolved after contacting with water, thereby obtaining a self-supporting oxide film. However, this kind of hydrolysis reaction has some disadvantages: (1) the process requires a long dissolution time; The experimental pseudocubic lattice constant of （2）Sr₃Al₂O₆ is about 3.961,961 Å, which limits the selection of the lattice constant of the target oxide film. (3) If crystal defects such as cracks occur in the preparation process, the integrity, effective size and properties of the system will be affected.

In order to improve these defects, a new water-soluble sacrificial layer material, ：Sr₄Al₂O₇, was successfully fabricated by pulsed laser deposition and molecular beam epitaxy, respectively^[8,9]. Sr₄Al₂O₇ is an allotrope of Sr₃Al₂O₆. Compared with Sr₃Al₂O₆, Sr₄Al₂O₇ has lower crystal symmetry [Fig. 1 (B)], so it shows better dynamic and thermal stability. Meanwhile, the in-plane pseudocubic lattice constant of Sr₄Al₂O₇ is about 3.896896 Å, which is closer to the perovskite system with the chemical formula of ABO₃, which is beneficial to the preparation of the heterojunction system of Sr₄Al₂O₇ and ABO₃; In addition, the Sr₄Al₂O₇ shows good "lattice elasticity", and the internal energy of the system increases slightly under the same strain [Fig. 1 (C)], which can transfer the lattice of the substrate material "completely" to the target film, and is conducive to the study of the film system under strain conditions. Finally, the density functional theory calculation results show that the bonding energy of the ：Sr₄Al₂O₇ and ABO₃ heterojunction interface is significantly higher than that of the Sr₃Al₂O₆ and ABO₃ heterojunction interface, which can effectively promote the formation of atomic-level complete interface. Based on the above advantages, a variety of free-standing oxide thin film samples prepared by using Sr₄Al₂O₇ as a water-soluble sacrificial layer material have the excellent characteristics of high quality, strong integrity, large sample size, and matching with the physical and chemical properties of epitaxial thin films.

By comparing the chemical formula of Sr₄Al₂O₇ with that of Sr₃Al₂O₆, it can be found that Sr₄Al₂O₇ has one more group of SrO, and more AlO₄^5- and Al₃O₁₀^11-.Combined with the characteristics of Sr — O bond and Al — O group, which are very soluble in water, it can be inferred that the larger the proportion of SrO in Sr-Al-O based water-soluble materials, the more the AlO₄^5- and Al₃O₁₀^11-, the better the water solubility. This speculation brings an open question: if the chemical formula of water-soluble materials in Sr-Al-O series is expressed as (SrO)_n+(Al₂O₃), whether there are higher order Sr_nAl₂O_3+n phase materials? It has better water solubility and lower crystal symmetry, and can prepare more different kinds of oxide free-standing thin films?

Currently, the in-plane pseudo-cubic lattice is quasi-tetragonal for both Sr₄Al₂O₇ and Sr₃Al₂O₆, which plays a significant advantage in the preparation of cubic or tetragonal (a = B) lattice oxide systems. However, there are still a large number of high-performance/functional oxide systems with orthorhombic (a > B) lattice structure, which require the substrate materials to have in-plane orthorhombic symmetry. Therefore, realizing the synthesis of higher order Sr_nAl₂O_3+n system is helpful to discover water-soluble materials of Sr-Al-O system with lower crystal symmetry, further realize the preparation and research of free-standing thin films of orthorhombic oxides, and promote their development and application.