Phase-transition behavior in high-pressure saline water systems is fundamental to understanding the deep hydrospheres of icy moons, the ice mantles of water-rich planets, and high-pressure ice physics. Compared with pure H₂O systems, salt ions not only modify the formation pathways of high-pressure ice phases such as ice Ⅵ and ice Ⅶ, but may also be redistributed as salt hydrates, salt-affected ice Ⅶ, or residual brines. In this review, we address the question of where salt ions reside in high-pressure aqueous systems by establishing a comparative framework based on two aspects: phase-transition pathway controls and evidence-assessment criteria. Concentration, water activity, pressure-temperature path, precursor state, and hydrate stability mainly govern the partitioning of salt ions among ice phases, hydrates, and residual fluids, whereas evidence type and evidence strength define the reliability and wording boundaries of different conclusions. On this basis, we review recent progress in representative systems including LiCl, NaCl, KCl, KBr, CaCl₂, and MgCl₂. Existing studies show that the LiCl system provides representative evidence for salt-affected ice Ⅶ formed through a non-equilibrium pathway; the NaCl system exhibits strong condition dependence and remains controversial with respect to extensive lattice incorporation; KCl/KBr and some NaCl systems preferentially form salt hydrates; the CaCl₂ system reveals a stepwise mechanism involving hydrate formation, hydrate decomposition, and subsequent perturbation of ice Ⅶ; and the MgCl₂ system indicates that the relationship between multiple hydrates and potential MgCl₂-ice Ⅶ still requires confirmation by continuous in situ experiments. It should be noted that this review focuses on representative chloride and halide systems for which relatively systematic high-pressure experimental data are available, whereas sulfate, carbonate, and multicomponent salt systems are discussed only briefly in the context of planetary implications and future research directions.

Metallic hydrogen, with its properties including room-temperature superconductivity and quantum fluidity, is known as the holy grail of high-pressure physics. However, since atomic metallic hydrogen requires pressures about 500 GPa, it has not been realized in experiments since its conception in 1935. To take advantage of properties the properties of metallic hydrogen in the future, it will be crucial to obtain it at ambient pressure. Current approaches to obtaining metallic hydrogen at low pressures rely on the “chemical precompression” in hydrides to induce metallization of hydrogen at low pressures, essentially identifying superconducting hydrides that can host the properties of metallic hydrogen. However, these superconducting hydrides currently lack distinct structural features, complicating the search for metallic hydrogen hosts. Here, we identify metallic hydrogen ligand compounds with hydrogen as the ligands as potential hosts for properties of metallic hydrogen at low pressures. The metallization of the non-bonding orbitals of the hydrogen ligands is a key criterion for determining whether a metallic hydrogen ligand compound can host metallic hydrogen properties. This article summarizes the main behaviors of hydrogen at ambient pressure, focusing on hydrogen ligand compounds at ambient pressure. Then, using a simple model of a one-dimensional hydrogen atom chain, we analyzed the causes of non-bonding orbital metallization and the physical picture of reduced stability pressure. The orbital characteristics of metallic hydrogen ligand compounds are then analyzed, highlighting their rules of superconductivity, topological properties, and the electronic structure that enable metallization. The analysis of metallic hydrogen ligand compounds presented in this article not only provides important structural information for future exploration of metal hydride superconductors but also provides an important theoretical foundation for realizing the properties of metallic hydrogen at ambient pressure.

The synthesis of the room-temperature superconductor LaSc₂H₂₄ represents a significant milestone in the field of superconductivity research. A central goal of subsequent studies is to lower the stabilization pressure required for hydrogen-rich superconductors, thereby establishing both theoretical foundation and technical pathway toward achieving low-pressure room-temperature superconductivity. This paper reviews recent advances in the prediction and experimental synthesis of hydride materials, with a focus on a promising strategy for realizing high-temperature superconductivity at reduced pressures—namely, H₂-molecular-typehydride. The superconducting mechanism dominated by molecular H₂ units is redefined, offering a new perspective for understanding phonon-mediated superconductivity. In H₂-molecular-type hydrides, a nearly free-electron gas behavior has been clearly observed. These delocalized electrons exhibit metallic bonding characteristics while retaining fragments of molecular hydrogen. This finding indicates that the essential condition for superconducting transition is the formation of a Fermi sea hosting Cooper pairs, rather than complete dissociation into atomic hydrogen. The generation mechanism of the free-electron gas in these materials can be effectively explained using a finite potential well model. The distinctive electronic properties of these compounds under high pressure, combined with enhanced electron-phonon coupling, establish a novel paradigm for designing low-pressure, high-temperature, and potentially room-temperature superconductors.