ON THE THERMODYNAMICS OF THE RHOMBOHEDRAL-CUBIC PHASE-TRANSITION OF THULIUM(III)OXIDE FLUORIDE

The rhombohedral(beta)-->cubic(alpha) phase transition of stoichiometr ic TmOF and the phase behaviour of samples of the analytical compositi on Tm(O,F), with 1.85 less than or equal to y less than or equal to 2. 10 were studied by quantitative differential thermal analysis (DTA) an d by X-ray powder diffraction analysis. It was found that the beta-->a lpha transition occurs at 808+/-3 K with an increment of the formula v olume of 0.5+/-0.05 Angstrom(3), and that alpha-TmOF decomposes in a p eritectoid reaction at ca. 915 K into a fluorite-related, presumably o rthorhombic phase of composition Tm(O,F)(1.93+/-0.02) and an orthorhom bic vernier-type phase of composition Tm9O8F11 with a=5.3765(5) Angstr om, b=49.402(5) Angstrom and c=5.4607(4) Angstrom. Tm(O,F)(1.93) is fo rmed by a eutectoid reaction of C-Tm2O3 with alpha-TmOF at ca. 900 K, An accurate determination of the enthalpy of the beta-->alpha transiti on of TmOF was found to be impossible because of the proximity of the temperatures of transition and decomposition. Therefore the enthalpy o f transition was determined indirectly by measuring the temperatures a nd enthalpies of transition of solid solutions Tm1-xCaxO1-xF1+x with 0 .017 less than or equal to x less than or equal to 0.062 by differenti al scanning calorimetry (DSC). It was found that both, T-tr and Delta( tr)H(T(tr))(0), depend linearily on the CaF2 content of the solid solu tions, and that the decomposition of the cubic alpha-modification is s uppressed by the incorporation of CaF2 into the TmOF lattice. The foll owing thermodynamic data relating to the beta-alpha phase transition o f stoichiometric TmOF could be derived by extrapolation to the formula TmOF: Delta(tr)H(808)(0)=5343+/-267 J mol(-1) and Delta(tr)S(808)(0)= 6.84+/-0.34 J mol(-1) K-1 By correlating the entropies of the beta-->a lpha transition with the temperatures of transition of ROF with R=Y, L a-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 Tm OF occupies a special position within the ROF series. (C) 1998 Elsevie r Science S.A.