Chemical bonding dictates drastic critical temperature difference in two seemingly identical superconductors

Though YB₆ and LaB₆ share the same crystal structure, atomic valence electron configuration, and phonon modes, they exhibit drastically different phonon-mediated superconductivity. YB₆ superconducts below 8.4 K, giving it the second-highest critical temperature of known borides, second only to MgB₂. LaB₆ does not superconduct until near-absolute zero temperatures (below 0.45 K), however. Though previous studies have quantified the canonical superconductivity descriptors of YB₆'s greater Fermi-level (E_f) density of states and higher electron-phonon coupling (EPC), the root of this difference has not been assessed with full detail of the electronic structure. Through chemical bonding, we determine low-lying, unoccupied 4f atomic orbitals in lanthanum to be the key difference between these superconductors. These orbitals, which are not accessible in YB₆, hybridize with π B-B bonds and bring this π-system lower in energy than the σ B-B bonds otherwise at E_f. This inversion of bands is crucial: the optical phonon modes we show responsible for superconductivity cause the σ-orbitals of YB₆ to change drastically in overlap, but couple weakly to the π-orbitals of LaB₆. These phonons in YB₆ even access a crossing of electronic states, indicating strong EPC. No such crossing in LaB₆ is observed. Finally, a supercell (the M k-point) is shown to undergo Peierls-like effects in YB₆, introducing additional EPC from both softened acoustic phonons and the same electron-coupled optical modes as in the unit cell. Overall, we find that LaB₆ and YB₆ have fundamentally different mechanisms of superconductivity, despite their otherwise near-identity.