On the rhombohedral-cubic phase transition of solid solutions ROFCaF2 with RGd, Tb and Dy

Abstract The rhombohedral-cubic phase transition typical of stoichiometric rare earth oxide fluorides of the formula type ROF was investigated for the ROF-based solid solution systems R 1− x Ca x O 1+ x F 1+ x , with RGd, Tb, Dy, by differential scanning calorimetry and by X-ray powder diffraction. It was found that the solubility limit of CaF 2 in these ROF is almost identical with x ⪷ 0.09 and that the transition temperatures decrease with increasing CaF 2 content. At x ≈ 0.9 this decrease is approximately 290 K, 250 K and 240 K for RGd, Tb temperatures decrease with increasing CaF 2 content. At x ≈ 0.9 this decrease is approximately 290 K, 250 K and 240 K for RGd, Tb and Dy, respectively, as compared to the transition temperatures of pure ROF. The formula volumes of the rhombohedral as well as the cubic solid solutions were determined as a function of x, and the structural implications of the observed trend is discussed for the rhombohedral modification. A partial tentative phase diagram is exemplarily proposed for Gd 1 − x Ca x O 1 − x F 1 − x .