  The results of electronic structure calculations for a variety of palladium
hydrides are presented. The calculations are based on density functional theory
and used different local and semilocal approximations. The thermodynamic
stability of all structures as well as the electronic and chemical bonding
properties are addressed. For the monohydride, taking into account the
zero-point energy is important to identify the octahedral Pd-H arrangement with
its larger voids and, hence, softer hydrogen vibrational modes as favorable
over the tetrahedral arrangement as found in the zincblende and wurtzite
structures. Stabilization of the rocksalt structure is due to strong bonding of
the 4d and 1s orbitals, which form a characteristic split-off band separated
from the main d-band group. Increased filling of the formerly pure d states of
the metal causes strong reduction of the density of states at the Fermi energy,
which undermines possible long-range ferromagnetic order otherwise favored by
strong magnetovolume effects. For the dihydride, octahedral Pd-H arrangement as
realized e.g. in the pyrite structure turns out to be unstable against
tetrahedral arrangement as found in the fluorite structure. Yet, from both heat
of formation and chemical bonding considerations the dihydride turns out to be
less favorable than the monohydride. Finally, the vacancy ordered defect phase
Pd3H4 follows the general trend of favouring the octahedral arrangement of the
rocksalt structure for Pd:H ratios less or equal to one.
