Temperature dependence of hydrogen site occupancy in Yttrium dihydride

Abstract

Neutron inelastic scattering and diffraction techniques were used to determine the temperature dependence of the octahedral site occupancy of hydrogen in YH 2.0 between 15 and 420 K. The results show an approximately constant fraction of the hydrogen atoms on octahedral sites up to 200 K followed by a monotonic decrease contrary to intuition and simple theory. This decrease in site disorder with increasing temperature is explained qualitatively by including the different effects of hydrogen at tetrahedral and octahedral sites on the metal lattice entropy and quantitatively by including hydrogen-hydrogen interactions.