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Enhanced Electron-Phonon Coupling and Superconductivity in Ba-Alloyed A15 LaH5.75

Recent experiments have established rare earth A15 type hydrides as a distinct family of high temperature superconductors that can be stabilized at significantly lower pressures compared to other superconducting hydrides. In particular, A15 type LaH5.75 was recently shown to be a high Tc superconductor. We have investigated a range of ternary substitutions and demonstrate that partial substitution of La by Ba stabilizes the A15 hydride lattice, with La0.75Ba0.25H5.75 identified as a stable compound. We calculate a superconducting transition temperature of approximately 183 K, almost double that of LaH5.75 at similar pressures. Ba typically acts as a 2 plus ion, while La is 3 plus. The reduced electron count disrupts the formation of H2 units and shifts the Fermi surface, leading to strongly enhanced electron phonon coupling. By contrast, substitution with Hf fails to produce any stable compounds. This work extends emerging alloy design principles in A15 hydride superconductors, demonstrating that doping can shift the Fermi level and tune the strength of the electron phonon coupling. These results provide concrete guidance for the experimental realization of new high Tc phases.