Phase transitions and isotope effects in (NH4)(2)HPO4 and (ND4)(2)DPO4

Phase behavior in (NH4)(2)HPO4 and (ND4)(2)HPO4 has been studied using ac c alorimetry and dielectric constant measurements. An unexpected reverse shif t has been observed in the phase transitions temperatures at T-1 = 174 K an d T-2 = 246 K for the proton compound to 147 K and 229 K, respectively, for the deuteron counterpart. A third phase transition near T-3 = 416 K shows no effect when substituting H for D. The phase transitions at low temperatu res are discussed in terms of ammonium reorientations at T-1 and T-2. Since a high generation of defects are induced in both compounds above room temp erature, as indicated by electrical conductivities measurements, we claim t hat, at 416 K, the observed first-order phase transition is triggered by th e coupling of these defects with the lattice distortion. The change of crys tal structure from monoclinic to orthorhombic symmetry that exhibits the 41 6 K transition influences the proton transport characteristics and, mainly, the mobile proton density which changes discontinuously through the phase boundary at T-3. However, there might be some dominant interaction for the transition to take place which has a negligible dependence on the mobile D (Hi) ion mass. (C) 1999 Elsevier Science B.V. All rights reserved.