Isotope effects on phase transitions in (ND 4) 2DPO 4

Abstract

High-resolution a.c. calorimetry was used to study the phase behaviour of (ND 4) 2DPO 4 and compare it with that of (NH 4) 2HPO 4 in the 70–450 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 as 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 no effect is observed on the T 3 transition temperature when substituting H for D. The phase transitions are discussed in terms of continuous (second-order) reorientations of the ND 4 + (NH 4 +) 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.