Heat capacities (1 to 108 K) and linear thermal expansivities (1 to 300 K) ofLuH0.148single crystals: Thermal relaxation effects and the pairing transition

Abstract

Previous heat capacity (C{sub p}) and linear thermal expansivity ({alpha}) data for the hexagonal {alpha}-LuH{sub x} and LuD{sub x} [LuH(D){sub x}] single crystal alloys (x=0,h0.005,h0.053) [C. A. Swenson, Phys. Rev. B {bold 53}, 3680 (1996)] have been extended to LuH{sub 0.148}. A feature (a transition) near 170 K in {alpha} vs {ital T} for LuH{sub 0.053} crystals is much more pronounced for the present results, with the {ital c}-axis ({ital a}-axis) data showing an almost 40{percent} (30{percent}) decrease (increase) in {alpha} on cooling below 170 K. This transition, which was associated with the pairing of H along the {ital c} axis in next-nearest-neighbor tetrahedral sites on opposite sides of a lutetium ion, is not clearly defined, however, and, after a change in temperature, is characterized by isothermal drifts in the sample length with time constants which are very small at 175 K but increase to 100 h at 144 K. The migration energy associated with the temperature dependence of these time constants [0.26(3) eV] is approximately one-half that which is associated with high-temperature bulk diffusion. The conclusion is that pair breakup (pairing) does not occur (is not completed) at a unique transition temperature when the alloy is warmed (cooled), butmore » is a thermally activated process, with the equilibrium fraction of paired H increasing with decreasing temperature, to achieve a saturation concentration below 140 K. The approach to pairing equilibrium for T{lt}175hK is diffusion limited. {copyright} {ital 1999} {ital The American Physical Society}« less