There are coupling and decoupling among the layers of condensed matter, and the critical phenomenon is the key to understand the decoupling between the layers. Under unconventional conditions such as sub-supercritical, the structure of the reactants shows different changes, and some reactions that are difficult to occur under conventional conditions can occur smoothly, thus forming rich metastable phase, singular phase, and superconducting equivalence. This paper focuses on the variation and evolution of multi‑level condensed‑matter structures under subcritical and supercritical hydrothermal conditions. Firstly, the structure of water molecules is briefly introduced. Structural transformations of reactant molecules and compositional‑structural features of products are then discussed from three perspectives: homoelemental systems (carbon), hetero‑elemental systems (Si‑O garnet), and multi‑elemental systems (Ca‑Si‑O tobermorite). The structural correlations during hydrothermal crystallization‑transformation are elucidated, aiming to provide certain insights for hydrothermal synthetic chemistry. Furthermore, condensed‑matter structures of atomic‑ and molecular‑scale devices are summarized, including solid‑state atomic‑scale pn junctions, neuronal ionic junctions, and the surface atomic‑rotation catalytic model. Finally, it is predicted that subcritical and supercritical hydrothermal synthesis technologies will offer practical inspirations for fields such as tunable design of “neuron‑mimicking” semiconductor materials, “subconscious” biological image sensing, and the construction of atomic‑rotation catalytic models.

Contents

1 Introduction

2 The Structure of Water Molecules Under Subcritical/Supercritical Conditions

3 Changes and Evolution of Multi-level Condensed Matter Structures Under Subcritical and Supercritical Hydrothermal Conditions

3.1 Condensed‑Matter Structures of Homoelemental Species

3.2 Condensed‑Matter Structures of Two Distinct Elements

3.3 Condensed‑Matter Structures of Multi‑Elemental Systems

3.4 Fabrication of Porous, Hierarchical and Heterostructured Architectures

4 Condensed Matter Structures of Devices at the Atomic and Molecular Scale

4.1 Structure of Solid Atomic-scale p-n Junctions

4.2 Network Structure of Neuronal-Ion Junctions

4.3 Atomic Rotation Catalysis Model on Material Surfaces

5 Conclusion and Outlook

5.1 Tunable Design of “Neuron‑Mimicking” Semiconductor Materials

5.2 “Subconscious” Biological Image Sensing and Ionic p‑n Junctions

5.3 Atomic‑Rotation Catalytic Model