Chemical binding energies of point defects in palladium doped with hydrogen and d impurities

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Abstract
Using an extra-orbital model within a generalized « spd » tight-binding approximation, we calculate the chemical binding energy of a pair of hydrogen atoms in palladium within the infinite dilution limit; we find that clusters of hydrogen cannot form in accordance with inelastic neutron scattering data in α-palladium hydride. The strong repulsive interaction between a pair of hydrogen impurities at distance a, where a is the host lattice parameter, seems to be related to the presence of an intermediate metallic site between the two impurities. Also we determine the chemical binding energies between a hydrogen atom and d substitutional impurities and we discuss our results in relation with recent Mössbauer experiments. Finally we calculate the chemical binding energy between two d impurities in palladium which we compare with experimental data on the enthalpy of formation