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

✍ Scribed by L. Topor; A. Navrotsky; Y. Zhao; D.J. Weidner

• Publisher: Elsevier Science
• Year: 1997
• Tongue: English
• Weight: 279 KB
• Volume: 132
• Category: Article
• ISSN: 0022-4596
• DOI: 10.1006/jssc.1997.7431

✦ Synopsis

The enthalpies of formation, heat content, heat capacity, and transition of NaMgF 3 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 -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.