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 measu red from 2 to 20 K with an adiabatic calorimeter. The results agreed e xcellently well with the previous data for T > 12 K. The heat capaciti es of Rb3H(SeO4)2 were larger than those of Rb3D(SeO4)2 in the tempera ture range 2.9 and 38 K. The excess heat capacity of Rb3H(SeO4)2 reach ed a maximum of 1.7 J K-1 mol-1 at 20 K. Its temperature dependence wa s analyzed with the Schottky heat capacity function. The low temperatu re limiting behavior of the excess heat capacity was reproduced well b y a two-level Schottky scheme whose excited level is distributed accor ding to a Gauss function centered at (52.5 +/- 0.3)R K with the full w idth at half maximum of (29.9 +/- 0.8)R K. Proton tunneling motion is discussed as a probable origin of the energy level splitting.