Phase diagrams of thE CaF 2- (Y, Ln) F 3 svstems I. Experimental

Abstract

The phase diagrams of the CaF 2- (Y, Ln) F 3 systems, where Ln is any one of the lanthanides except Pm and Eu, were studied by thermal and X-ray phase analysis methods. The investigations were made from 800 (600)°C up to the melting temperature. The oxygen concentration of the specimens was checked after thermal treatment. For the systems with Ln = Ce, Pr, Tb, Ho, Tm and Lu the fusibility data have been obtained for the first time. Phases with the following structure types are present in the systems investigated: (a) solid solutions Ca 1− x (Y, Ln) x F 2+ x with the fluorite structure type, which undergo ordering in the systems with Ln = HoLu, Y; (b) non-stoichiometric phases (Y, Ln) 1− y Ca y F 3− y with the tysonite (LaF 3) structure type, which undergo ordering in the systems with Ln = TbLu, Y; (c) solid solutions with the α-UO 3 structure type, based on α modifications (Y, Ln) F 3 (Ln = ErLu); and (d) modifications (Y, Ln) F 3 (Ln = GdLu) of the β-YF 3 type. Successive transitions, from solid solutions of the components of the system to the berthollide phases and the stoichiometric compound, were found for phases with the tysonite structure type. The maxima on fusion curves of non-stoichiometric phases were noticed in Sm, Gd, Tb, Ho and Er for Ca 1− x - (Y, Ln) x F 2− x and for Ln = PrLu in (Y, Ln) 1− y Ca y F 3− y . The coordinates of non-variant points for equilibria of the liquid-solid type and also the equations describing the dependence of the lattice parameters of nonstoichiometric phases on the composition are given for the systems investigated.