Measured Relative Enthalpy of Anhydrous Crystalline Aluminum Trifluoride, AlF, from 273 to 1173 °K and Derived Thermodynamic Properties from 273 to 1600 K.

The enthalpy of a high-purity sample of anhydrous crystalline aluminum trifluoride, AlF, relative to that at 0 °C (273.15 °K), was precisely measured with an ice calorimeter and a "drop" method at 18 temperatures starting at 50 °C and proceeding in 50-deg steps to 900 °C (1173.15 °K). Thirty additional enthalpy measurements between 450 and 453 °C revealed a gradual transition. A simple general relation for the progress of transition when impurity is in solid solution is derived. The relation fits the observed transition data and indicates a first-order transition temperature of 455 °C (728 °K). X-ray powder patterns on the sample, measured in the Crystallography Section of the NBS, established the existence of a phase transition by showing not only the known hexagonal structure at room temperature (even after violent quenching from above the transition temperature region) but a new, simple-cubic structure at 570 °C (843 °K). The smooth heat-capacity curve formulated from the data merges very smoothly with that representing published precise low-temperature data. The common thermodynamic properties were derived, and are tabulated at and above 273.15 °K, with extrapolation up to 1600 °K.