Structure of the hydrogen arrangement around a positive muon in α′-PdHx☆

Abstract Positive muon μ + spin rotation measurements have been performed in a single-crystal PdH 0.74 sample between 20 and 150 K. The μ + depolarization rate σ has been measured as a function of crystal orientation on a magnetic field. With increasing temperature a strong reduction in the σ anisotropy occurs, whereas only a moderate decrease in average depolarization rate is observed. It is assumed that protons and muon occupy octahedral interstitial sites in the palladium lattice. The data can be fitted by a decreasing proton occupancy of the octahedral sites nearest the μ + with increase in temperature. The proton occupancy of the remaining octahedral sites at all temperatures, as well as the occupancy of the proton sites nearest the μ + at 20 K, is compatible with the bulk nominal hydrogen concentration x = 0.74. It appears that at 20 K the μ + would stay at a location with “regular” nearest-neighbour proton occupancy, but that at higher temperatures diffusion or reordering would allow the μ + to find an environment with less nearest-neighbour protons.