ON THE RHOMBOHEDRAL-CUBIC PHASE-TRANSITION OF SOLID-SOLUTIONS ROF-CAF2 WITH R=GD, TB AND DY

The rhombohedral-cubic phase transition typical of stoichiometric rare earth oxide fluorides of the formula type ROF was investigated for th e ROF-based solid solution systems R1-xCaxO1-xF1+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 CaF, content. At x approximate to 0.9 this decrease i s approximately 290 K, 250 K and 240 K for R=Gd, Tb and Dy, respective ly, as compared to the transition temperatures of pure ROF. The formul a volumes of the rhombohedral as well as the cubic solid solutions wer e determined as a function of x, and the structural implications of th e observed trend is discussed for the rhombohedral modification. A par tial tentative phase diagram is exemplarily proposed for Gd1-xCaxO1-xF 1+x. (C) 1997 Elsevier Science S.A.