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

Abstract For seven decades, when referring to A-15 superconductors, we meant metallic A 3 B 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). Nb 3 Ge exhibited the highest superconducting transition temperature, T c = 23K, among these alloys. One of these alloys, Nb 3 Sn, 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 La 4 H 23 phase compressed at P = 118 GPa. Despite the fact that La 4 H 23 has much lower T c in comparison with the near-room-temperature superconducting LaH 10 phase ( T c ,zero = 250K at P ∼ 200 GPa) discovered by Drozdov et al (2019 Nature 569 531), La 4 H 23 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 La 4 H 23 . In this paper, we analyzed available experimental data measured in La 4 H 23 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 La 4 H 23 phase are similar to those in MgB 2 , cuprates, pnictides, and the near-room-temperature superconductors H 3 S and LaH 10 . To the memory of Martin J. Ryan, man and scientist who taught EFT the intricacies of X-ray diffraction.