High-Pressure Hydrides Overview

• High-pressure hydrides are compounds formed by incorporating hydrogen atoms into metal lattices at pressures above 10 GPa, featuring expanded stoichiometry and unique bonding arrangements.
• Experimental methods like diamond-anvil cell synthesis and laser heating, combined with ab initio calculations, enable phase diagram mapping and stabilization of novel polyhydride frameworks.
• Advanced spectroscopic techniques and electronic structure analyses reveal tunable superconducting properties and potential applications in hydrogen storage and chemical precompression.

High-pressure hydrides are compounds in which hydrogen atoms are incorporated into the crystal lattice of metals or molecular networks under extreme thermodynamic conditions, typically exceeding 10 GPa and often reaching or surpassing 200 GPa. These materials exhibit a remarkable expansion of hydrogen stoichiometry and structural diversity compared to ambient-pressure binary hydrides, stabilizing novel frameworks, polyhydride anions, and hydrogenic sublattices. High-pressure hydrides are of paramount interest in condensed-matter physics, chemistry, and materials science because of their tunable electronic properties—including superconductivity at (or near) room temperature—and their relevance to hydrogen storage and "chemical precompression" paradigms.

1. Thermodynamic Stability and Phase Diagrams

High-pressure hydrides are often accessed through diamond-anvil cell (DAC) synthesis, laser heating, and guided by ab initio calculations. The phase diagram of a given metal–hydrogen system is mapped by constructing the pressure-dependent convex hull of formation enthalpies:

$$\Delta H_f(P) = E[MH_n](P) - E[M](P) - (n/2) E[H_2](P)$$

For multicomponent or electrochemically modulated cases, the Gibbs free-energy landscape is evaluated using

$$\Delta G(P, \Phi) = G[MH_n](P) - G[M](P) - n\,\mu_H(\Phi)$$

where μ_H(Φ) is the chemical potential of hydrogen as a function of applied electrochemical potential. High-pressure studies extend the hydride stoichiometry well beyond ambient limits: for example, in the Y–H and La–H systems, stable phases sequentially appear as pressure increases (YH₂ → YH₃ → YH₄ → YH₆ → YH₉/YH₁₀ above 150 GPa; LaH₂ → LaH₃ → LaH₄/LaH₆/LaH₉/LaH₁₀ >75–150 GPa) (Phuthi et al., 25 Sep 2025, Laniel et al., 2022). In alkali and alkaline-earth systems, polyhydrides such as NaH₃ and NaH₇ are stabilized above 30–40 GPa and 2000 K, confirmed by both x-ray diffraction and theoretical structure searches (Struzhkin et al., 2014).

Transition-metal hydrides exhibit similar complexity: in the Ir–H system, IrH₃ (P6₃mc) forms above 5 GPa, giving way to IrH₂ (Ibam/Cmcm) at higher pressures; in Fe–H, FeH₅, FeH₆, FeH₇, and FeH₈ structures span 150–300 GPa, some metallic, others insulating (Zarifi et al., 2018, Zaleski-Ejgierd, 2013). Ternary and molecular hydrides further enrich this landscape, as exemplified by ammonia polyhydrides (NH₁₀, NH₁₁, NH₂₄) (Wang et al., 19 May 2024), K–Y–H phases (Chen, 2021), and Li–Si–H hypervalent frameworks (Liang et al., 2020).

2. Structural Motifs and Hydrogen Sublattices

At extreme pressure, hydrogen adopts bonding arrangements inaccessible at ambient conditions. High-pressure hydrides often feature:

• Clathrate frameworks: "Sodalite"-like or related cages in CaH₆ (Im–3m), YH₆, LaH₁₀, stabilized by full electron donation from electropositive metals. Each metal ion sits at the center of a H₂₄/H₂₈ polyhedral cage, with H–H separations down to 1.2–1.3 Å (Wang et al., 2012, Bi et al., 2018, Laniel et al., 2022).
• Polyhydride/anionic moieties: Linear H₃⁻ units (NaH₇, FeH₇), square H₄ blocks (CaH₆), H₇⁻ and H₁₀⁻ chains (amyad in NH₁₁, NH₂₄), and discrete molecular H₂, interstitial or chain-like, providing both electronic and vibrational versatility (Struzhkin et al., 2014, Wang et al., 19 May 2024, Zarifi et al., 2018).
• Hypervalent coordination: Main-group elements (e.g., Si in Li–Si–H) form layered SiH₅⁻, trigonal-prismatic SiH₆²⁻, and incorporate classical and nonclassical bonding motifs beyond 8-N rule limits (Liang et al., 2020).

Metal frameworks may range from close-packed rare-earth sublattices to layered and one-dimensional arrangements, with hydrogen content (and metal valence) dictating site occupation and symmetry transitions. Discrete jumps in H content, with plateaus at specific stoichiometries, are characteristic; solid-solution behavior is comparatively rare under pressure (Phuthi et al., 25 Sep 2025, Laniel et al., 2022).

3. Electronic Structure, Charge Transfer, and Bonding

Compression dramatically enhances charge transfer from metals to hydrogen. Bader charge analysis in Y–H phases reveals a monotonic increase of ≈0.15 e per H atom (YH₂–YH₉) and ≈0.40 e per H atom (YH₁₀) over 0–200 GPa, reflecting increasing ionic character in M–H bonds and the stabilization of extended H frameworks (Phuthi et al., 25 Sep 2025). Metallic character predominates in most binary and ternary hydrides at megabar pressure, supported by density of states projections showing strong H–s contribution at the Fermi level, as in LaH₁₀, YH₆, SbH₄, and related systems (Fu et al., 2015, Laniel et al., 2022).

Hypervalency, multi-center bonding, and the presence of nonclassical anions enable structural and compositional flexibility. In systems such as CaH₆, electrons donated by the cation reside in degenerate cage states, producing electronic degeneracy and dynamic Jahn–Teller instabilities (Wang et al., 2012). Multicenter bonding motifs, such as those in NaH₇ or SiH₆²⁻, are stabilized by the enthalpic gain from pressure-enhanced overlap and charge delivery from alkali/alkaline-earth ions (Struzhkin et al., 2014, Liang et al., 2020).

4. Superconductivity, Electron–Phonon Coupling, and Theoretical Frameworks

Many high-pressure hydrides are conventional phonon-mediated superconductors, with transition temperatures (T_c) determined by robust (ω_log, λ) combinations:

$$T_c = \frac{\omega_{log}}{1.2} \exp \left[ -\frac{1.04(1+\lambda)}{\lambda - \mu^*(1+0.62\lambda)} \right]$$

where λ is the EPC constant, ω_log is the logarithmic average phonon frequency, and μ* is the Coulomb pseudopotential (0.10–0.13). Parameters are computed from first-principles linear-response theory or full Eliashberg equations (Wang et al., 2021, Saha et al., 2022, Wang et al., 2012).

Record high T_c’s are approached in clathrate binary hydrides (H₃S, 203 K at 155 GPa; LaH₁₀, 260 K at 188 GPa; YH₁₀, 305 K at 250 GPa; CaH₆, 220–235 K at 150 GPa) and in select ternary phases (Li₂MgH₁₆ and LaBH₈) (Wang et al., 2021, Bi et al., 2018, Zhu et al., 18 Nov 2024). Ternary clathrates Y₂CdH₁₈, Y₂InH₁₈, Ca₂SnH₁₈ exhibit T_c well above 110 K at 180–250 GPa, extending the chemical design latitude for high-T_c hydrides (Zhu et al., 18 Nov 2024). Non-trivial molecular and chain networks in ammonia polyhydrides (NH₁₀, NH₁₁, NH₂₄) provide T_c up to 179 K at 300 GPa (Wang et al., 19 May 2024).

EPC in high-T_c hydrides is typically dominated by mid-frequency M–H wagging/bending and H-network stretching modes, with high-frequency H₂ or H₃ vibrons only weakly coupled (Fu et al., 2015, Ma et al., 2015). Anharmonicity and structural quantum effects can appreciably renormalize phonon frequencies and reduce predicted T_c by tens of kelvin (Wang et al., 2021). For transition-metal-rich phases (e.g., FeH₅, IrH₂), λ is small (<0.2), precluding high-T_c states (Zarifi et al., 2018, Zaleski-Ejgierd, 2013).

5. Experimental Techniques, Phase Identification, and Quantification

Diamond-anvil cell synthesis and in-situ characterization—using x-ray diffraction, Raman/IR spectroscopy, and resistivity—are core methods for accessing and mapping high-pressure hydride stability. Accurate hydrogen stoichiometry is critical: recent development of microcoil NMR techniques in DACs enables direct quantification of H content from x ≈ 0.1 to x ≥ 7, over 30–200 GPa, independent of lattice volume or presence of metallic backgrounds (Meier et al., 2022). This has exposed sample heterogeneity, phase coexistence, and composition variability—especially in rare-earth systems such as La–H, which hosts seven structurally distinct phases in a single preparation (Laniel et al., 2022).

Identification of specific phases relies on single-crystal XRD and comparison with DFT-relaxed archetypes. In La–H, for example, Fm–3m LaH₃ (50 GPa), I4/mmm LaH₄+δ (140–155 GPa), Im–3m LaH₆+δ (150 GPa), P6₃/mmc LaH₉+δ (140–176 GPa), and Fm–3m LaH₁₀+δ (140–176 GPa) are individually resolved, with the additional challenge that hydrogen site occupancy exhibits partial filling (+δ), shifting the onset of cage formation and T_c (Laniel et al., 2022).

Experimental caveats encompass unintended synthesis routes (e.g., silane decomposition liberating H₂ to react with DAC components, yielding false positives for hydride phases) (0907.2128), ambiguous electronic transport (resistance drops without magnetic confirmation), and the continual need for explicit demonstration of bulk Meissner flux expulsion (Hirsch et al., 2021).

6. Trends, Chemical Design Principles, and Materials Mapping

High-pressure hydride stability and superconductivity exhibit distinct trends with chemical composition, electronegativity, and hydrogen content:

• Hydrogen-rich stoichiometries are increasingly stabilized at higher pressures, especially for electropositive metals (alkali, alkaline-earth, rare-earth), due to favorable charge transfer and Madelung energy contributions (Phuthi et al., 25 Sep 2025, Fu et al., 2015, Bi et al., 2018, Saha et al., 2022).
• Extended hydrogen frameworks (clathrate cages, polymeric chains, 2D networks) maximize hydrogenic bandwidth and the density of states at the Fermi level, which correlates with large λ and high T_c (Wang et al., 2012, Zhu et al., 18 Nov 2024).
• Phase transitions with pressure often occur in discrete H/M ratios rather than as continuous solid solutions, linked to the stabilization of local bonding motifs at specific electron counts (Phuthi et al., 25 Sep 2025).
• Descriptors such as Bader charge on H, H–H bond length (1.0–1.8 Å), quasi–bulk modulus (500–700 GPa), and H-centered DOS serve as efficient computational filters for high-throughput materials discovery (Saha et al., 2022).

Ambient-pressure electronegativity difference (χ_M – χ_H) is predictive: highly negative values promote ionic M–H bonding and H–H motifs, intermediate values (0 to −0.2) favor superconducting frameworks with high DOS(E_F) (Fu et al., 2015).

7. Outstanding Challenges and Future Directions

Despite exceptional progress, several open issues remain in the paper of high-pressure hydrides:

• Superconductivity confirmation: While resistive transitions and isotope effects are routinely observed, definitive evidence of bulk Meissner expulsion is rare or contested for many candidates, and spurious signals arise from mixed or minority phases, especially in "superhydride" systems (Hirsch et al., 2021, Wang et al., 2021, Laniel et al., 2022).
• Anharmonicity and non-Migdal corrections: Many hydrides possess λ ≫ 2, casting doubt on calculations that neglect vertex corrections. Advanced SCDFT and fully anharmonic approaches are needed (Wang et al., 2021).
• Precise H-position determination: Diffraction and NMR advances are closing the gap, yet atomic-scale understanding of hydrogen sublattices—especially light atom disorder or partial site occupation—remains limited (Laniel et al., 2022, Meier et al., 2022).
• Design of ternary/quaternary phases: Ternary hydrides (Y₂CdH₁₈, Y₂InH₁₈, Ca₂SnH₁₈, KYH₈) and molecular polyhydrides (NH₁₀, NH₂₄, Li₂MgH₁₆) are emerging as promising routes to lower-pressure, high-T_c materials, as evidenced by targeted computational screening (Zhu et al., 18 Nov 2024, Chen, 2021, Wang et al., 19 May 2024).
• Experimental synthesis and metastability: Many theoretically stable phases have yet to be isolated experimentally, with kinetic barriers and sample containment presenting substantial practical obstacles.

Ongoing advances in computation, in situ detection, and mechanistic understanding are poised to expand the compositional and structural repertoire of high-pressure hydrides, enabling the rational exploration and ultimately the realization of room-temperature superconductors stabilized by extreme conditions (Wang et al., 2021, Saha et al., 2022, Bi et al., 2018).