Heat capacities (1 to 108 K) and linear thermal expansivities (1 to 300 K)of LuH0.148 single crystals: Thermal relaxation effects and the pairing transition

Previous heat capacity (C-p) and linear thermal expansivity (alpha) data fo r the hexagonal alpha-LuHx and LuDx [LuH(D)(x)] single crystal alloys (x = 0, 0.005, 0.053) [C. A. Swenson, Phys. Rev. B 53, 3680 (1996)] have been ex tended to LuH0.148. A feature (a transition) near 170 K in alpha vs T for L uH0.053 crystals is much more pronounced for the present results, with the c-axis (a-axis) data showing an almost 40% (30%) decrease (increase) in alp ha on cooling below 170 K. This transition, which was associated with the p airing of H along the c axis in next-nearest-neighbor tetrahedral sites on opposite sides of a lutetium ion, is not clearly defined, however, and, aft er a change in temperature, is characterized by isothermal drifts in the sa mple 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 dep endence of these time constants [0.26(3) eV] is approximately one-half that which is associated with high-temperature bulk diffusion. The conclusion i s that pair breakup (pairing) does not occur (is not completed) at a unique transition temperature when the alloy is warmed (cooled), but is a thermal ly activated process, with the equilibrium fraction of paired H increasing with decreasing temperature, to achieve a saturation concentration below 14 0 K. The approach to pairing equilibrium for T< 175 K is diffusion limited.