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

The enthalpies of formation, heat content, heat capacity, and transiti on of NaMgF3 perovskite have been determined. A new high temperature c alorimetric methodology has been applied for the enthalpy of formation measurements. The heat content and heat of formation measurements wer e done using a ''hybrid calorimeter'' built recently in our laboratory . For the heat capacity measurements, a Netzsch 404 high-temperature d ifferential 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 transi tion at 1051 K. The presence of one peak supports a recent X-ray study which concluded that the crystal structure transforms from the orthor hombic Pbnm phase directly to the cubic Pm3m phase. The enthalpy of tr ansformation is 2.3+/-0.03 kJ/mol. The heat content values measured by transposed temperature drop calorimetry are in agreement with those c alculated by graphical integration of the experimental heat capacity d ata obtained by DSC. (C) 1997 Academic Press.