ISOTOPE EFFECTS ON PHASE-TRANSITIONS IN (ND4)(2)DPO4

High-resolution a.c. calorimetry was used to study the phase behaviour of (ND4)(2)DPO4 and compare it with that of (NH4)(2)HPO4 in the 70-45 0 K temperature range. The temperature dependence of the heat capacity is similar in both salts, but there is an unexpected reverse shift in the phase transition temperatures at T-1 = 174 K and T-2 = 246 K for the proton compound to 147 K and 229 K, respectively, for the deuteron counterpart. A third phase transition is also verified near T-3 = 416 K in both salts, which shows a thermal hysteresis of about 15 K and n o effect is observed on the T-3 transition temperature when substituti ng H for D. The phase transitions are discussed in terms of continuous (second-order) reorientations of the ND4+ (NH4+) tetrahedra over two preferred directions at T-1 and T-2 and a discontinuous (first-order) generation of mobile D+ (H+) defects at T-3. (C) 1997 Elsevier Science Ltd.