  A recent experimental work on palladium hydrides suggested that metastable
structures with hydrogen atoms occupying tetrahedral sites could lead to
superconductivity above 50 K, a huge increase compared to the 10 K critical
temperature of the stable structure with all hydrogen atoms occupying
octahedral sites. By generating many structures with hydrogen atoms randomly
occupying the octahedral and tetrahedral sites of the face-centered cubic
lattice and calculating their energy at different theoretical levels from first
principles, we determine that metastable structures with partial or full
occupation of tetrahedral sites are possible, even when the ionic quantum
zero-point energy and anharmonicity are included in the calculations.
Anharmonicity is crucial in palladium hydrides when hydrogen atoms occupy
octahedral sites and, in fact, makes the structure with full octahedral
occupation the ground state. Despite the metastable existence of structures
with full or partial tetrahedral sites occupation, the superconducting critical
temperature is reduced with the number of tetrahedral sites occupied. Our
calculations discard that the occupation of tetrahedral sites can increase the
critical temperature in palladium hydrides.
