The Y <sub> 2-x </ Sub> Of Er <sub> X </ Sub> Mo <sub> 4 </ Sub> O <sub> 15 </ Sub> And Y-<sub> 2-x </ Sub > Sm <sub> The X </ Sub> W <sub> 3 </ Sub> O <sub> 12 <

To search for more negative thermal expansion(NTE) materials, better understand mechanisms of NTE and study their physical properties, new series of rare earth compounds Y_2-xEr_xMo₄O₁₅ and Y_2-xSm_xW₃O₁₂ were prepared successfully by the conventional solid state reaction technique. Their structures, thermal expansion properties and other physical properties were studied systematically by ambient X-ray diffraction, high temperature X-ray diffraction, thermogravimetric analysis etc. The main research contents and results are as follows:(1) A new series of rare earth compounds Y_2-xEr_xMo₄O₁₅(x=0.0-2.0) were prepared successfully. Their structures have been studied using powder X-ray diffraction(XRD) and were found to have the topological similarity to the structures of typical NTE materials ZrW₂O₈ and ZrV₂O₇. The lattice parameters a, b, e,βand the unit cell volume V decrease linearly with increasing Er content. Thermal expansion properties have been investigated by high-temperature X-ray diffraction. The results indicate that the compounds of this series have similar normal positive thermal expansion behavior. According to the analysis of diffraction data, the common edge O14-O23 should be the main resistance for NTE behavior and the weak bond M02-O14 should be responsible for the positive thermal expansion in these compounds. By comparison, the effect of band W1-O3 in ZrW₂O₈ is discussed. As a result, a possible model for the phase transition of ZrW₂O₈ is put forward.(2) A new series of rare earth solid solutions Y_2-xSm_xW₃O₁₂(x=0.0-0.4) were synthesized successfully and studied by means of powder X-ray diffraction. All samples crystallize in an orthorhombic space group Pnca. The lattice parameters a, b, c and unit-cell volume V of Y_2-xSm_xW₃O₁₂ increase monotonically with increasing Sm content. The structure of pure Sm₂W₃O₁₂ was also studied by powder X-ray diffraction. The hygroscopic property of Y_2-xSm_xW₃O₁₂ at room temperature was studied by TG analysis and high temperature X-ray diffraction. The role of water molecules, in determining the change of crystal structures and thermal expansion behavior has been discussed. The thermal expansion behavior of the Y_2-xSm_xW₃O₁₂ compositions has been studied by high temperature X-ray diffraction. In the studied temperature range, 200-600℃, the compounds of this series show strong negative thermal expansion along all three axes(a, b and c). By adjusting the Sm content in Y_2-xSm_xW₃O₁₂, compounds with variable thermal expansion coefficient can be prepared. The prepared compound becomes less pronounced in negative thermal expansion and stores less water content with increasing Sm content at room temperature. The effect of the intermingle of Sm content has been speculated in this paper.