The A-15-type superconducting hydride La4H23: a nanograined structure with low strain, strong electron-phonon interaction, and a moderate level of nonadiabaticity

For seven decades, when referring to A-15 superconductors, we meant metallic A₃B alloys (where A is a transition metal, and B is group IIIB and IVB elements) discovered by Hardy and Hulm (1953 Phys. Rev. 89 884). Nb₃Ge exhibited the highest superconducting transition temperature, T_c = 23K, among these alloys. One of these alloys, Nb₃Sn, is the primary material in modern applied superconductivity. Recently, Guo et al (2024 Natl Sci. Rev. nwae149, https://doi.org/10.1093/nsr/nwae149) extended the family of superconductors where the metallic ions are arranged in the beta tungsten (A-15) sublattice by observation of T_c,zero = 81K in the La₄H₂₃ phase compressed at P = 118 GPa. Despite the fact that La₄H₂₃ has much lower T_c in comparison with the near-room-temperature superconducting LaH₁₀ phase (T_c,zero = 250K at P ∼ 200 GPa) discovered by Drozdov et al (2019 Nature 569 531), La₄H₂₃ holds the record for the highest T_c within the A-15 family. Cross et al (2024 Phys. Rev. B 109 L020503) confirmed the high-temperature superconductivity in compressed La₄H₂₃. In this paper, we analyzed available experimental data measured in La₄H₂₃ and found that this superconductor exhibits a nanograined structure, 5.5 nm ⩽ D ⩽ 35 nm, low crystalline strain, |ε| ⩽ 0.003, strong electron–phonon interaction, 1.5 ⩽ λ_e-ph⩽ 2.55, and a moderate level of nonadiabaticity, 0.18 ⩽ Θ_D/T_F ⩽ 0.22 (where Θ_D is the Debye temperature, and T_F is the Fermi temperature). We found that the derived Θ_D/T_F and T_c/T_F values for the La₄H₂₃ phase are similar to those in MgB₂, cuprates, pnictides, and the near-room-temperature superconductors H₃S and LaH₁₀.To the memory of Martin J. Ryan, man and scientist who taught EFT the intricacies of X-ray diffraction.