A15-type Hydrides: High-Pressure Superconductors

• A15-type hydrides are hydrogen-rich metallic compounds crystallizing in a cubic A15 structure, where hydrogen acts as a major structural component.
• They show high-Tc superconductivity, with transition temperatures reaching up to 118K, driven by strong electron–phonon coupling and unique hydrogen cage architectures.
• High-pressure synthesis and advanced computational screenings validate their dynamic, elastic, and kinetic stability, paving the way for novel superconducting materials.

A15-type hydrides are metallic hydrogen-rich compounds structurally analogous to classic β-tungsten (A15) intermetallics, but incorporate hydrogen either as a major “B”-site component (as in La₄H₂₃) or as a network former in ternary hydrides (as in YSbH₆). Their discovery adds a significant new class of high-pressure superconductors exhibiting phonon-mediated high-$T_c$ behavior, novel hydrogen cage topologies, and metastable stability regimes. Unlike classical A15 alloys (e.g., Nb₃Sn), these hydrides can reach superconducting transition temperatures ($T_c$) well above liquid nitrogen across a range of particle morphologies and pressures, placing them among the highest-$T_c$ A15 phases known. A15 hydrides display a rich synergy of structural motifs, electron–phonon coupling, and kinetic/thermodynamic metastability, often bridging the design concepts of classical A15 superconductors and superhydrides such as LaH₁₀ or YH₆.

1. Structural Motifs and Crystallography

A15-type hydrides crystallize in the cubic A15 (β-W or Cr₃Si-type) structure, space group $Pm\bar{3}n$ (No. 223), featuring a three-dimensional network of hydrogen cages enveloping metal chains or polyhedra. In prototypical binary hydrides like La₄H₂₃, the structure is characterized by La ions on the 6c Wyckoff sites arranged as intersecting orthogonal chains, while the 23 hydrogen atoms per formula unit fill both polyhedral cages around La and bridge La–H–La motifs. The refined lattice constant is approximately $a = 6.19\,$Å at 95 GPa for La₄H₂₃ (Cross et al., 2023, Guo et al., 2023), compared to $a = 3.6659\,$Å at 50 GPa for the ternary YSbH₆ (Caussé et al., 22 Dec 2025).

In YSbH₆, the A15-type structure accommodates Y at the simple cubic 1a position and Sb at the body-center 1b position, each enveloped by a 12-vertex hydrogen cage. The hydrogen sublattice forms edge-sharing polyhedra with characteristic planar six-membered rings, creating a robust three-dimensional network (Caussé et al., 22 Dec 2025). For both binary and ternary A15 hydrides, typical H–H bond lengths in the cages range from $1.0$ to $1.3$ Å, reflecting strong compression and the promotion of high-frequency phonon modes (Cross et al., 2023, Talantsev et al., 2024).

2. Synthesis, Phase Stability, and Microstructure

A15 hydrides require high-pressure synthesis, typically achieved in diamond-anvil cells (DACs) with pressures in the range $90$–$120$ GPa for La₄H₂₃ (binary) and $20$–$120$ GPa for YSbH₆ (ternary) (Cross et al., 2023, Caussé et al., 22 Dec 2025). Precursor materials such as La or LaH₃ react with hydrogen or hydrogen donors (e.g., ammonia borane) under laser heating (often $T\sim 1000$–$2000$ K) to form the desired phase. The formation of A15 La₄H₂₃ is robust in a narrow window around $95$–$120$ GPa, while YSbH₆ exhibits metastable behavior with decreasing enthalpy above the convex hull as pressure increases (e.g., $\Delta E_\mathrm{hull}=108\,\mathrm{meV/atom}$ at $50$ GPa, dropping to $26\,\mathrm{meV/atom}$ at $120$ GPa) (Caussé et al., 22 Dec 2025).

Microstructural studies using in situ synchrotron x-ray diffraction and Williamson–Hall analysis confirm that A15 hydrides, particularly La₄H₂₃, possess nanograined morphologies (grain size $D$ in the range $5.5$–$35$ nm) with crystalline strain below $0.003$, which is atypically low for high-pressure phases (Talantsev et al., 2024). This nanocrystallinity may enhance phonon softening at grain boundaries without introducing significant pair-breaking defects.

3. Electronic Structure and Superconductivity

The electronic structure of A15 hydrides is characterized by high densities of states at the Fermi level with significant hydrogen-derived contributions (e.g., $\sim 30\%$ in La₄H₂₃ at $120$ GPa) (Guo et al., 2023). The primary mechanism of superconductivity is phonon-mediated pairing, supported by robust electron–phonon coupling constants ($\lambda_\mathrm{e-ph}$) in the strong-coupling regime. For La₄H₂₃, extracted values range from $1.5$ to $2.55$ as a function of pressure and synthesis history (Talantsev et al., 2024), while YSbH₆ shows $\lambda\approx 1.95$ at $50$ GPa (Caussé et al., 22 Dec 2025).

Table: Superconducting parameters for representative A15 hydrides

Compound	$P$ (GPa)	$\lambda_\mathrm{e-ph}$	$\omega_\mathrm{log}$ (K)	$T_c$ (K)
YSbH₆	50	1.95	670	118
La₄H₂₃	95	1.5–2.55	650 (calc.)	81–105
ScSiH₆	90	n.r.	n.r.	116
CaSbH₆	45	n.r.	n.r.	65
MgPH₆	135	n.r.	n.r.	148

$n.r.$: not reported (Caussé et al., 22 Dec 2025, Guo et al., 2023, Talantsev et al., 2024).

In La₄H₂₃, $T_c$ (onset) reaches up to $105$ K at $118$ GPa (Guo et al., 2023), with experimental confirmation of $T_c$ up to $95$ K at $95$ GPa after annealing (Cross et al., 2023), and $T_{c,zero}=81$ K at $118$ GPa (Talantsev et al., 2024). YSbH₆ achieves $T_c=118$ K at $50$ GPa as predicted by isotropic Migdal–Eliashberg theory (Caussé et al., 22 Dec 2025). Both compounds exceed by far the upper bound of $T_c$ for classic metallic A15s (e.g., Nb₃Ge, $T_c=23$ K).

4. Dynamical, Elastic, and Kinetic Stability

Phonon calculations indicate that A15 hydrides achieve dynamical stability above threshold pressures: for La₄H₂₃, no imaginary modes exist above $120$ GPa (Guo et al., 2023); for YSbH₆, dynamical stability holds from $20$ to $120$ GPa, with all phonon frequencies positive above $20$ GPa (Caussé et al., 22 Dec 2025). Elastic constants for YSbH₆ at both $20$ and $50$ GPa satisfy Born's criteria for a cubic phase (e.g., $C_{11}=535$ GPa, $C_{12}=290$ GPa, $C_{44}=120$ GPa at $50$ GPa) (Caussé et al., 22 Dec 2025).

Molecular dynamics simulations reveal kinetic trapping of the cubic phase, with hydrogen diffusion within H cages negligible at 300 K up to 0.5 ns and the cage motif resilient up to 700 K, consistent with long-lived metastability and high kinetic barriers (Caussé et al., 22 Dec 2025). This persistence under non-equilibrium conditions is critical for experimental realization and the preservation of superconducting properties upon decompression.

5. Magnetotransport and Exotic Normal-State Phenomena

Magnetotransport in A15 hydrides, particularly La₄H₂₃, shows both expected and unconventional behavior. Superconducting upper critical fields ($\mu_0 H_{c2}(0)$) determined by Ginzburg–Landau and Werthamer–Helfand–Hohenberg models range from $24$ to $63$ T, with coherence lengths $\xi_0$ on the order of $2.7$–$3.7$ nm (Guo et al., 2023, Cross et al., 2023). A notable observation in La₄H₂₃ is large negative magnetoresistance (up to $-55\%$ at $4.5$ K and $68$ T), a pseudogap region below $40$ K (non-Fermi liquid, quasi-linear $R(T)$), and sign changes in $dR/dT$ under decompression (Guo et al., 2023). This behavior parallels phenomena observed in underdoped cuprates and signals a breakdown of conventional Migdal–Eliashberg transport, referring to the emergence of a strange-metal or pseudogap phase in the normal state.

6. Thermodynamic Context: Comparison with Classical and Superhydride Systems

A15-type hydrides bridge the superconducting property space between classic metallic A15s and high-$T_c$ superhydrides. In Uemura and Pietronero identification diagrams (plotting $T_c/T_F$ and $\Theta_D/T_F$ against $\lambda_\mathrm{e-ph}$), La₄H₂₃ and related hydrides occupy the regime of cuprates, pnictides, $H_3S$, and LaH₁₀ ($T_c/T_F\approx 2$–$3\times10^{-3}$, $\Theta_D/T_F\approx 0.016$–$0.022$), while classic A15 intermetallics have much lower ratios ($T_c/T_F < 10^{-4}$, $\Theta_D/T_F < 10^{-3}$) (Talantsev et al., 2024). This suggests that high-pressure hydrides, though phonon-mediated, realize “unconventional” energy scale ratios typically associated with non-BCS mechanisms.

No hydrogen-rich ternary phase (H/M ≥ 3) lies on the convex hull at moderate pressures. In the Y–Sb–H system, the highest H/M ratio on the hull is $4/3$ (in Y₂SbH₄) at 120 GPa; YSbH₆ is within $26$ meV/atom of the hull at 120 GPa (Caussé et al., 22 Dec 2025). This proximity defines a practical boundary between “synthesizable metastable” and “inaccessible” hydrides.

7. Methodological Advances: High-Throughput and Machine-Learning Screening

A15 hydrides have emerged as the result of multi-stage, high-throughput computational searches. For YSbH₆, screening proceeds from prototype A15–ABH₆ surveys to evaluation of $T_c$ over coarse grids, followed by detailed convex hull construction using ephemeral data-derived potentials trained on $10^4$–$10^5$ DFT calculations (Caussé et al., 22 Dec 2025). Synthetic feasibility is judged on the basis of dynamic, elastic, thermodynamic, and kinetic stability, with final candidates exhibiting proximity to the convex hull, absence of soft phonon modes, and demonstrable kinetic trapping (Caussé et al., 22 Dec 2025). A plausible implication is that further adoption of such screening, especially with machine-learning-accelerated structures, will yield new A15 hydrides with tunable properties at more experimentally accessible pressures.

1. "Metastability and high-Tc superconductivity in A15-type ternary hydride YSbH₆ at moderate pressure" (Caussé et al., 22 Dec 2025)
2. "High-temperature superconductivity in A15-type La4H23 below 100 GPa" (Cross et al., 2023)
3. "Large negative magnetoresistance and pseudogap phase in superconducting A15-type La$_4$H$_{23}$" (Guo et al., 2023)
4. "A-15 type superconducting hydride $La_4H_{23}$: Nanograined structure with low strain, strong electron-phonon interaction, and moderate level of nonadiabaticity" (Talantsev et al., 2024)