Thermal properties of XMF 3: cubic perovskites I. Heat capacity of NH 4MgF 3 and NH 4CdF 3

Abstract

The heat capacities of NH 4MgF 3 and NH 4CdF 3 have been determined in the range 5 to 350 K by d.c. adiabatic calorimetry. First-order phase transitions at T c = (107.5±0.1) K and T c = (331.2±0.1) K were found for both compounds, corresponding to the cubic-to-tetragonal structural transition, together with a reorientation of NH 4 + groups. For both anomalies an entropy of ΔS R ≈ ln 3 has been obtained. The thermodynamic functions at 298.15 K are: C p, m R S m o R {H m o (T)−H m o (0)} (R· K) − {G m o (T)−H m o (0)} RT NH 4MgF 3 15.88 17.55 2916.3 7.764 NH 4CdF 3 20.70 21.01 3276.5 10.024 An excess of heat capacity over the phonon contribution appears above 50 K which is discussed in terms of the hindered rotations of NH 4 + in a point-charge electrostatic potential. Moreover, the origin of the structural transition at T c is discussed.