On the thermodynamics of the rhombohedral-cubic phase transition of thulium(III)oxide fluoride

Abstract

The rhombohedral(β)→cubic(α) phase transition of stoichiometric TmOF and the phase behaviour of samples of the analytical composition Tm(O,F) y with 1.85≤ y≤2.10 were studied by quantitative differential thermal analysis (DTA) and by X-ray powder diffraction analysis. It was found that the β→α transition occurs at 808±3 K with an increment of the formula volume of 0.5±0.05 Å 3, and that α-TmOF decomposes in a peritectoid reaction at ca. 915 K into a fluorite-related, presumably orthorhombic phase of composition Tm(O,F) 1.93 ± 0.02 and an orthorhombic vernier-type phase of composition Tm 9O 8F 11 with a=5.3765(5) Å, b=49.402(5) Å and c=5.4607(4) Å. Tm(O,F) 1.93 is formed by a eutectoid reaction of C-Tm 2O 3 with α-TmOF at ca. 900 K. An accurate determination of the enthalpy of the β→α transition of TmOF was found to be impossible because of the proximity of the temperatures of transition and decomposition. Therefore the enthalpy of transition was determined indirectly by measuring the temperatures and enthalpies of transition of solid solutions Tm 1- x Ca x O 1- x F 1+ x with 0.017≤ x≤0.062 by differential scanning calorimetry (DSC). It was found that both, T tr and Δ tr H T(tr) 0, depend linearily on the CaF 2 content of the solid solutions, and that the decomposition of the cubic α-modification is suppressed by the incorporation of CaF 2 into the TmOF lattice. The following thermodynamic data relating to the β→α phase transition of stoichiometric TmOF could be derived by extrapolation to the formula TmOF: Δ tr H 808 0=5343±267 J mol −1 and Δ tr S 808 0=6.84±0.34 J mol −1 K −1. By correlating the entropies of the β→α transition with the temperatures of transition of ROF with R=Y, La–Tm it was found that the entropies for R=La–Eu and Tm are clustering around 6.6±0.4 J mol −1 K −1 and those for R=Gd–Er and Y around 8.5±0.5 J mol −1 K −1, independently of the cationic radii, and that TmOF occupies a special position within the ROF series.