A CALORIMETRIC STUDY OF THE DEUTERATION EFFECT ON THE PHASE-BEHAVIOR IN RB3H(SEO4)2 AND RB3D(SEO4)2 .2. HEAT-CAPACITIES AT VERY-LOW TEMPERATURES

The heat capacities of Rb3H(SeO4)2 and Rb3D(SeO4)2 crystals were measured from 2 to 20 K with an adiabatic calorimeter. The results agreed excellently well with the previous data for T > 12 K. The heat capacities of Rb3H(SeO4)2 were larger than those of Rb3D(SeO4)2 in the temperature range 2.9 and 38 K. The excess heat capacity of Rb3H(SeO4)2 reached a maximum of 1.7 J K-1 mol-1 at 20 K. Its temperature dependence was analyzed with the Schottky heat capacity function. The low temperature limiting behavior of the excess heat capacity was reproduced well by a two-level Schottky scheme whose excited level is distributed according to a Gauss function centered at (52.5 +/- 0.3)R K with the full width at half maximum of (29.9 +/- 0.8)R K. Proton tunneling motion is discussed as a probable origin of the energy level splitting.