The phase transition, hygroscopicity, and thermal expansion properties of Yb_(2-x)Al_xMo_3O_(12)

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-xAlxMo3012 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 orthorhonlbic phase transition temperature of A12Mo3012 can be reduced by partial substitution of A13＋ by Yb3＋, and the Yb2-zAlxMo3012 （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 A13＋ for Yb3＋ in Yb2Mo3012 decreases its hygroscopicity, and the linear thermal expansion coefficients after complete removal of water species are measured to be -9.1 x 10-6/K, -5.5 x 10-6/K, 5.74 x 10-6/K, and 9.5 x 10 6/K for Ybl.sAlo.2（MoO4）3, Yb1.6Alo.4（MoO4）3, Ybo.4All.6（Mo04）3, and Ybo.2Al1.8（MoO4）3, respectively.

Negative Thermal Expansion of (ZrMg)xY2-2xMo3O12 Ceramics with Low Hygroscopicity

(ZrMg)xY2-2xMo3O12(x=0.0,0.1,0.2,0.3,0.4,0.5,0.6,0.7,0.8,0.9,and 1)ceramics have been synthesized to obtain less hygroscopicity and negative thermal expansion.With increasing the substitution of(ZrMg)^6+(ion radius 7.2×10^-11 m)for Y3+(ion radius 9×10^-11 m),the crystal water are reduced obviously.The linear thermal expansion coefficient is improved with increasing the content of(ZrMg)^6+.The material shows near zero thermal expansion(-0.12×10^-6 K^-1,430-870 K)with x=0.7.Meanwhile,ZrMgMo3O12 shows low non-hygroscopicity and negative thermal expansion,but low softening temperature.After substituting amount of Y^3+for(ZrMg)^6+in ZrMgMo3O12(x=0.8),the softening temperature increases remarkably(750 K to 830 K)and it presents near zero thermal expansion.

A Negative Thermal Expansion Material of ZrMgMo_(3)O_(12)

A material with the formula ZrMgMo_(3)O_(12) having negative thermal expansion is presented and characterized.It is shown that ZrMgMo_(3)O_(12) crystallizes in an orthorhombic symmetry with space group Pnma(62)or Pna21(33)and exhibits negative thermal expansion in a large temperature range(αl=-3.8×10^(-6) K^(-1) from 300 K to 1000 K by x-ray diffraction andαl=-3.73×10^(-6) K^(-1) from 295 K to 775 K by dilatometer).ZrMgMo_(3)O_(12) remains the orthorhombic structure without phase transition or decomposition at least from 123 K to 1200 K and is not hygroscopic.These properties make it an excellent material with negative thermal expansion for a variety of applications.