Raman Spectroscopic Study On The Structure And Phase Sransition Of A₂(MoO₄)₃

Most of the solid crystals expand with the temperature increasing in general; on the contrary, there are several compounds which contract as the temperature rises. This kind of abnormal thermal expansion behavior is called the negative thermal expansion (NTE). In recent years, the study of such materials has attracted significant attention due to scientific curiosity and technological interest. One of the important applications is to composite these materials with other positive thermal expansion materials to tailor the thermal expansion to a desired value.Of the NTE materials, those with formula A2(MO4)3, where "A" is a trivalent cation and M is W or Mo, being anisotropic in thermal expansion coefficient, have received attention. The A2(MO4)3 compounds may crystallize either in a monoclinic or orthorhombic structure depending on the "A" cation size. Only the orthorhombic ones exhibit significant negative thermal expansion behavior. The orthorhombic structure has an open framework structure with M-O-M'linkages which can accommodate for transverse thermal vibrations responsible for negative thermal expansion. In this thesis, we first study the structure and phase transitions of A2(MoO4)3 compounds by Raman spectroscopy, where A=Al, Cr, Fe, Y and Yb, and then try to decrease the phase transion temperature of Al2(MoO4)3 through partial or complete replace of Al by Zr and Mg. The main results obtained are as follows.(1) Materials of Y2(MO4)3 and Yb2(MO4)3 are strong hygroscopic at room temperature. Their formulas are determined by thermal analysis to be Y2(MoO4)3.3.27H2O, Yb2(MoO4)3.2.84H2O, Y2(WO4)3.3.17H2O and Yb2(WO4)3.4.27H2O, respectively. Raman spectroscopic study shows that the presence of water species hinders every type of motion of the corner shared polyhedral, making these materials expand. They have three typical Raman band in room temperature. With the temperature increasing, Raman peaks become sharp and split into several bands. This indicates that the water species have a direct interaction with the vibration mode.(2) Fe2(MoO4)3, Cr2(MoO4)3 and Al2(MoO4)3 crystallize in monoclinic structure. The monoclinic modifications in A2(MO4)3 family have an edge sharing structure, which are more densely packed and cannot accommodate for transverse thermal vibrations. With the temperature increasing, these materials change to orthorhombic. The measured the phase transition temperatures are 484K,674K and 786K for Al2(MoO4)3, Cr2(MoO4)3 and Fe2(Mo04)3, respectively. The characteristic Raman bands at 1028cm-1,998cm-1 and 992cm-1 for monoclinic Al2(MoO4)3, Cr2(MoO4)3 and Fe2(MoO4)3, respectively, disappear after phase transition to orthorhombic.(3) The high phase transition temperatures (much higher than room temperature) of these materials indicate that they cannot be used as NTE materials for applications. We tried to reduce the phase transition temperature of Al2(MoO4)3 by replacing Al with Zr and Mg. The formula can be written as (Al2X(ZrMg)1-x)(MoO4)3. Raman spectroscopic analyses reveal that (Al2X(ZrMg)1-x)(MoO4)3 crystallize in orthorhombic structure when x≦0.5. The monoclinic structural to orthorhombic phase transition temperature of Al(ZrMg)o.s(Mo04)3 is well below 160K. And the sample could keep stable from 173K to 873K on the basis of temperature dependence of the Raman spectra and DSC analyses.