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

For seven decades by A-15 superconductors we meant metallic A₃B alloys (where A is a transition metal, and B is groups IIIB and IVB element) discovered by Hardy and Hulm (Phys. Rev. 89, 884 (1953)). Nb3Ge exhibited the highest superconducting transition temperature, Tc = 23 K, among these alloys. One of these alloys, Nb₃Sn, is primary material in modern applied superconductivity. Recently Guo et al (arXiv:2307.13067) extended the family of superconductors where the metallic ions arranged in the beta tungsten (A-15) sublattice by observation of Tc,zero = 81 K in La₄H₂₃ phase compressed at $P = 118 GPa$. Despite the La₄H₂₃ has much lower Tc in comparison with near-room-temperature superconducting LaH₁₀ phase (Tc,zero = 250 K at $P = 200 GPa$) discovered by Drozdov et al (Nature 569, 531 (2019)), the La₄H₂₃ holds the record high Tc within A-15 family. Cross et al (Phys. Rev. B 109, L020503 (2024)) confirmed the high-temperature superconductivity in the compressed La₄H₂₃. In this paper, we analyzed available experimental data measured in La₄H₂₃ and found that this superconductor exhibits nanograined structure, 6 nm < D < 27 nm, low crystalline strain < 0.005, high electron-phonon coupling constant, 1.5 < λₑ₋ₚₕ < 2.7, and moderate level of the nonadiabaticity ΘD/TF. We found that derived ΘD/TF and Tc/TF values for the La₄H₂₃ phase are similar to the ones in cuprates, pnictides, and near-room-temperature superconductors H₃S and LaH₁₀, which implies that the La₄H₂₃ phase falls to unconventional superconductors band in the Uemura plot.