Abstract: Materials with the formula Yb_2-xAl_xMo₃O₁₂ (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 Yb_2-xAl_xMo₃O₁₂ solid solutions crystallize in a single monoclinic phase for 1.7≤ x≤ 2.0 and in a single orthorhombic phase for $0.1\le x\le 0.4$, 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 Yb_2-xAl_xMo₃O₁₂ (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 Yb_1.8Al_0.2(MoO₄)₃, Yb_1.6Al_0.4(MoO₄)₃, Yb_0.4Al_1.6(MoO₄)₃, and Yb_0.2Al_1.8(MoO₄)₃, respectively.

Key words: negative thermal expansion material, phase transition, hygrosopicity, Raman spectroscopy