THERMOCHEMISTRY OF FLUORIDE PEROVSKITES - HEAT CAPACITY, ENTHALPY OF FORMATION, AND PHASE TRANSITION OF NAMGF3

摘要

The enthalpies of formation, heat content, heat capacity, and transition of NaMgF3 perovskite 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 a lambda-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 orthorhombic Pbnm phase directly to the cubic Pm3m phase. 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. (C) 1997 Academic Press. [References: 38]