Role of tetrahedrally coordinated dopants in palladium hydrides on their superconductivity and inverse isotope effect

Absorption of hydrogen by palladium causes PdH to become superconducting below . Due to the presence of one octapore and two tetrapores per each Pd atom, it is believed that of PdH should increase further. Here, using ab initio calculation we show that (i) H placed in tetrapores of PdH induces a wide optical gap in the phonon density of states, which significantly reduces the electron-phonon coupling, and that (ii) the energetically preferable octapores filled by H enable the 9 K superconductivity only. This scenario may close a long-standing problem of the high- palladium hydrides. Moreover, simulating the pore population by H and D, within ab initio molecular dynamics, we are able to explain the inverse isotope effect in the framework of the Bardeen-Cooper-Schrieffer theory.