Neutron Diffraction Study of Temperature-Dependent Properties of Palladium Containing Absorbed Hydrogen

Abstract

Neutron diffraction techniques have been employed to study the hydrogen-atom configuration in a single-phase sample of beta-PdH at several selected temperatures. The suggested low-temperature ($T\ensuremath{\ll}55\ifmmode^\circ\else\textdegree\fi{}$K) structure of this compound is one which conforms to the space group $R\overline{3}m$, which differs from the high temperature ($T\ensuremath{\gg}55\ifmmode^\circ\else\textdegree\fi{}$K) structure [$\mathrm{Fm}3m$]. The low-temperature structure is formed by a partial migration of hydrogen atoms from octahedral to nearby tetrahedral crystallographic sites in the face-centered cubic palladium lattice. Approximate values of the root-mean-square vibrational amplitude of the hydrogen atoms have been determined to be 0.25 \AA{} ($T=293\ifmmode^\circ\else\textdegree\fi{}$K) and 0.17 \AA{} ($T=4.2\ifmmode^\circ\else\textdegree\fi{}$K). The anomalous behavior observed in measurements of the temperature dependence of the electrical resistivity and heat capacity of this compound is explained by the transfer of the hydrogen atoms between the lattice sites.