The phase transition, hygroscopicity, and thermal expansion properties of Yb_2−<i>x</i>Al_<i>x</i>Mo₃O₁₂

Abstract

Materials with the formula Yb2−xAlxMo3O12 (x = 0.1, 0.2, 0.3, 0.4, 0.5, 0.7, 0.9, 1.0, 1.1, 1.3, 1.5, and 1.8) were synthesized and their structures, phase transitions, and hygroscopicity investigated using X-ray powder diffraction, Raman spectroscopy, and thermal analysis. It is shown that Yb2−xAlxMo3O12 solid solutions crystallize in a single monoclinic phase for 1.7 ≤ x ≤ 2.0 and in a single orthorhombic phase for 0.0 ≤ x ≤ 0.4, and exhibit the characteristics of both monoclinic and orthorhombic structures outside these compositional ranges. The monoclinic to orthorhombic phase transition temperature of Al2Mo3O12 can be reduced by partial substitution of Al3+ by Yb3+, and the Yb2−xAlxMo3O12 (0.0 < x ≤ 2.0) materials are hydrated at room temperature and contain two kinds of water species. One of these interacts strongly with and hinders the motions of the polyhedra, while the other does not. The partial substitution of Al3+ for Yb3+ in Yb2Mo3O12 decreases its hygroscopicity, and the linear thermal expansion coefficients after complete removal of water species are measured to be −9.1 × 10-6/K, −5.5 × 10-6/K, 5.74 × 10-6/K, and 9.5 × 10-6/K for Yb1.8Al0.2(MoO4)3, Yb1.6Al0.4(Mo04)3, Yb0.4Al1.6(MoO4)3, and Yb0.2Al1.8(MoO4)3, respectively.