Rare earth substitution in scheelite

Substitution of calcium in scheelite by lanthanum and samarium with or without sodium was studied in the temperature range between 600° and 1150°C using quenching techniques. In the systems CaWO4La2(WO4)3 and CaWO4Sm2(WO4)3, complete series of solid solutions form above 1020°C, but in each system there is a phase boundary separating the tetragonal scheelite and the monoclinic defect scheelite, while in the systems CaWO4NaLa(WO4)2 and CaWO4NaSm(WO4)2, immiscibility gaps disappear at 825°C, and they do not have any detected phase change in the solid solutions.In the ternary system CaWO4NaLa(WO4)2La2(WO4)3, at 750°C, a three-phase region consisting of tetragonal scheelite, tetragonal disordered scheelite, and monoclinic defect scheelite occupies the central portion of the phase diagram. At 900°C, scheelite with either disordered scheelite or defect scheelite is the major phase assemblage; and at 1025°C, there are only one-phase regions, the tetragonal solid solution transforming gradually to the monoclinic solid solution with increasing amount of La2(WO4)3.