Deuterium occupation of tetrahedral sites in palladium

The long-standing controversy over the occupation by hydrogen of tetrahedral interstices in palladium has been addressed experimentally and theoretically. Using the highest resolution neutron powder diffractometer available, diffraction profiles were recorded from single-phase samples obtained by loading Pd with deuterium in situ at 310 degrees C, at D(2) pressures up to 90 bar. Rietveld profile analysis showed that a model including tetrahedral occupancy was necessary to properly fit the experimental diffraction profiles. The maximum absolute tetrahedral occupancy was found at a deuterium-to-metal atomic ratio of 0.6, where about one-third of all D atoms were in tetrahedral sites. At the lowest and highest D concentration, the tetrahedral fraction approached zero. The energy of formation was calculated, based on density-functional theory, for numerous configurations of octahedral and tetrahedral interstitials in a supercell, which modeled stoichiometries Pd(8)H(n) such that n=1,2,...8. For Pd(8)H(3), the minimum formation energy was found with 1-2 tetrahedral atoms. For all other stoichiometries, the minimum formation energy was 0-1 tetrahedral atoms. Thus, the calculations are in excellent qualitative agreement with experiment and support the reality of tetrahedral occupancy.