Thermochemistry of Fluoride Perovskites: Heat Capacity, Enthalpy of Formation, and Phase Transition of NaMgF3

Abstract

The enthalpies of formation, heat content, heat capacity, and transition of NaMgF3perovskite have been determined. A new high temperature calorimetric methodology has been applied for the enthalpy of formation measurements. The heat content and heat of formation measurements were done using a “hybrid calorimeter” built recently in our laboratory. For the heat capacity measurements, a Netzsch 404 high-temperature differential scanning calorimeter (DSC) has been used. The enthalpy of formation from the binary fluorides at 298 K is −15.76±2.46 kJ/mol, and that from the elements is −1716.51±2.8 kJ/mol. The measured heat capacities between 313 and 1153 K show aλ-anomaly with a transition at 1051 K. The presence of one peak supports a recent X-ray study which concluded that the crystal structure transforms from the orthorhombicPbnmphase directly to the cubicPm3mphase. The enthalpy of transformation is 2.3±0.03 kJ/mol. The heat content values measured by transposed temperature drop calorimetry are in agreement with those calculated by graphical integration of the experimental heat capacity data obtained by DSC.