On the rhombohedral-cubic phase transition of solid solutions ROF-CaF2 with R=Gd, Tb and Dy

Article abstract of DOI:10.1016/S0925-8388(97)00093-5

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-xCa_xO_1-xF_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 CaF₂ in these ROF is almost identical with x < 0.09 and that the transition temperatures decrease with increasing CaF₂ 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-xCa_xO_1-xF_1+x.