Palladium deuteride: neutrons versus DFT

We recently compared the atomic volume of H in PdHx, calculated with popular DFT schemes assuming purely octahedral (O) interstitial occupancy, to that obtained by neutron diffraction for x<0.8. Significant disagreement was revealed between theory/modelling based on H/D O-only occupancy and experiment. The DFT-calculated rate of lattice expansion for tetrahedral (T) occupancy is significantly higher than for O, suggesting a choice between two controversial possibilities: either there is no T occupancy and "standard" DFT - as applied to Pd-H2 for 50 years - does not work, or there is partial T occupancy at the 10-20% level. This proposal aims to explore D concentrations x>0.6. This will extend the range of available experimental data from neutrons and so bolster the comparison with DFT. Secondly, the measurements should reveal the mix of O and T (if any) occupancy at high x values.