The ?-? high-temperature transition of CaF2

Abstract

Calcium fluoride (fluorite) is considered one of the most prominent among the alkaline-earth halides. First evidence of a high-temperature transition of CaF2 was gained from heat-content measurements [1] which suggested that the low-temperature face-centered cubic phase ("c~"-CaFz) is transformed to a high-temperature form of unknown structure ("fl"-CaFz) at 1151 ~ There exists an empirical rule, with only few exceptions, that high-temperature transitions involve an increase in structural symmetry. Since CaFa belongs, however, to the highly symmetrical space group Fm3m, this observation posed the interesting question which changes in crystal symmetry might be expected. Based on the data of [1] a phase transition of second order was inferred [2], thus involving only small structural alterations. Recent investigations concerned with the high-temperature transitions of halides with fluorite structure on a broad scope [3, 4] seemed to support this view. A high-temperature X-ray study at 1 atm has been conducted. First structural data of/~-CaF2 obtained at 1200 ~ are presented. For the diffraction experiment, a Guinier high-temperature X-ray camera (Enraf-Nonius, type Guinier-Lenne) was employed. The powder samples were taken from a natural crystal (Sarntal, Tyrolia, Italy) of low impurity content. Its lattice parameter at room temperature is in good agreement with standard data [5]: asample =