HIGH-TEMPERATURE X-RAY-DIFFRACTION STUDY OF THE RHOMBOHEDRAL-CUBIC PHASE-TRANSITION OF ROF WITH R=Y, LA, PR, ND, SM-ER

The rhombohedral-cubic phase transition of ROF with R=Y, La, Nd, Sm-Er and the expansion coefficients of both modifications were studied by the high-temperature Guinier X-ray powder diffraction method. It was found that a constant formula volume increment of 0.37 +/- 0.04 Angstrom(3), which is independent of the cationic size, is characteristic of this transition. The formula volumes depend practically linearly on temperature, and the volume expansion coefficients, which are generally larger for the cubic than for the rhombohedral modification, vary irregularly across the R(3+) series. For the cases of YOF, LaOF, NdOF, SmOF, GdOF and ErOF it could be shown that experimental data for the transitional volume changes, entropies and pressure dependence of the transition temperatures obey the Clausius-Clapeyron equation within the accuracy limits of the data.