Calculation of oxygen distribution coefficients of RF3 (R = La, Gd) fluorides with the tysonite structure during their crystallization from a melt

Using the method of modified cryoscopy, the thermodynamic distribution coefficients of oxygen k0 in LaF3 and α-GdF3 with a tysonite structure (sp. gr. )were calculated from the fusibility diagrams of condensed systems RF3–R2O3 (R = La, Gd). The calculated coefficients k0 are 1.02 and 1.12 for lanthanum and gadolinium trifluorides, respectively. The values of the coefficients k0 satisfy the condition k0 1, which confirms the formation of maxima in the fusibility curves of tysonite solid solutions tys- RF3–2xOx. For LaF3, the proximity of the distribution coefficient to k0 = 1 corresponds to an almost uniform distribution of oxygen in the volume of the crystallized fluoride melt. Knowledge of oxygen distribution coefficients during crystallization from a melt is important for choosing a strategy for crystallophysical purification of trifluorides RF3 from oxygen impurities and obtaining oxofluorides tys-RF3–2xOx with a given impurity distribution.