The Synthesis And Thermal Expansion Property Of Yb_2-xAl_xMo₃O₁₂

In recent years, the negative thermal expansion (Negative thermal expansion, referred to as NTE) materials have received increasing attention as the development of modern society. One type of the negative thermal expansion materials A2M3O12 due to their rich polymorphism and interesting physical and chemical properties has much important potential uses in many areas, such as optical, electronic, medicinal and mechanical applications. A3+can be almost any trivalent cations capable of accepting octahedral coordination to oxygen and M6+can be Mo6+or W6+in the compounds. It's reported that the linear thermal expansion coefficient is a function of 'A'cation size. Different A cation sizes make the A2(MO4)3 compounds crystallize in different structures:monoclinic structure or orthorhombic structure. Only the orthorhombic compounds exhibit negative thermal expansion behavior. As the 'A' cation size decreases the thermal expansion coefficient also becomes less negative. Some of A2M3O12 family with an orthorhombic structure (A=Y, Er, Yb and Lu) are highly hygroscopic and exhibit negative thermal expansion after the complete removal of water molecules while some of A2M3O12 family with an monoclinic structure (A= Al, Fe, Cr) at room temperature. With the increasing of temperature, these materials change to orthorhombic.In order to search for more negative thermal expansion (NET) materials, explain mechanisms of NET better and study other physical properties, we prepared a series of compounds Yb2-xAlxMo3O12 by the conventional solid state reaction technique successfully. With ambient X-ray diffraction, Raman spectroscopy and thermogravimetric analysis etc, we studied their thermal expansion properties and some other physical properties systematically. The main research contents and conclusions are as follows:Materials with formulaYb2-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) have been synthesized and their structures, phase transitions and hygroscopicity have been investigated by 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 both characters of 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+. The orthorhombic Yb2-xAlxMo3O12 (0.0<x<0.4) ones are hydrated at room temperature and contain two kinds of water species, one of those interacts strongly with and hinders the motions of the polyhedra while the other does not. Partial substitution of Al3+for Yb3+in Yb2Mo3O12 decreases its hygroscopicity.