Phase Transition And Thermal Expansion Properties In Near-zero Thermal Expansion Materials With General Formula ABMo₃O₁₂

Most materials change their dimensions when the temperature changes.Usually,they expand on heating and contract on cooling,and this is known as positive thermal expansion（PTE）.This thermal induced volume change in a confined space could lead to considerable stress even failure of devices.On the other hand,materials with opposite thermal properties,namely negative thermal expansion（NTE）,are investigated widely.The excitement about NTE materials is that,in principle,they could compensate for PTE,fundamentally reducing even eliminating thermal stress when the devices are exposed to thermal shocks.In recent years,the family of A₂M₃O₁₂（A=transition metal or a mixture of tetravalent and bivalent cations,M=W,Mo）have been extremely attractive,for whose stable NTE goes over a wide temperature range and chemical flexibility makes it possible possessing variation of linear CTE from low positive to large negative values by varying the type and content of the substitutional cation.In this thesis,the phase transition temperature,near-zero thermal expansion and anisotropy of some materials with a general formula ABMo₃O₁₂（A=tetravalent cations,B=bivalent cations）are investigated in details.1、Near-zero thermal expansion（near-ZTE）and phase transitions in HfMg_1-xZn_xMo₃O₁₂Solid solutions of HfMg_1-xZn_xMo₃O₁₂2 with near-ZTE are successfully synthesized by solid state reaction and the effects of Zn²⁺incorporation on the phase formation,phase transition,thermal expansion,vibrational properties and micro-morphologies are investigated by XRD,Raman spectroscopy,dilatometry and SEM.It is shown that（i）single phase formation is only possible for x≤0.5,otherwise,additional phases of HfMo₂O₈ and ZnMoO₄ generate;（ii）The phase transition temperature from monoclinic to orthorhombic structure of the single phase HfMg_1-xZn_xMo₃O₁₂2 can be well tailored,which increases with the content of Zn²⁺.And itcrystallizes in an orthorhombic structure for x≤0.4 and in a monoclinic structure for x=0.5 at room temperature;（iii）The incorporation of Zn²⁺leads to an pronounced reduction in the positive expansion of the b-axis and an enhanced NTE in c-axes,making the Zn-containing materials exhibit near-zero thermal expansion over a wide temperature range and lower anisotropy in thermal expansion in the orthorhombic phase;（iv）Replacement of Mg²⁺by Zn²⁺breaks the local electronic equilibrium around the MoO₄ tetrahedron and weakens the Mo-O bonds,leading to obvious red shifts of all the Mo-O stretching modes with increasing the content of Zn²⁺due to obviously higher electronegativity of Zn²⁺than Mg²⁺;（v）The incorporation of Zn²⁺improves sintering property of samples,minimizing the possible contributions of extrinsic effects such as pores,which is preferred for most applications.The mechanisms of these phenomena are also discussed.2、Design,synthesis and Near-zero Thermal Expansion of HfMg_1-xMn_xMo₃O₁₂Solid solutions of HfMg_1-xMn_xMo₃O₁₂2 were designed to show near-zero thermal expansion and successfully synthesized by solid state reaction with optimal conditions.The effects of Mn²⁺incorporation on the thermal expansion,phase transition and vibrational properties are investigated by dilatometry,XRD and Raman spectroscopy.The results show that the replacement Mg²⁺with Mn²⁺makes the materials have smaller thermal expansion coefficient and show near-zero thermal expansion properties;The co-doping of Mg²⁺&Mn²⁺ions makes the phase transition temperature of HfMg_1-xMn_xMo₃O₁₂2 tunable,and the phase transition temperature increases with the content of Mn²⁺;Substitution of Mg²⁺by Mn²⁺could break the local electronic equilibrium around the MoO₄ tetrahedron,due to differences in ionic radius and electronegativity of Mg²⁺and Mn²⁺,weaking the Mo-O bonds,leading to obvious red shifts of all the Mo-O stretching modes with increasing the content of Mn²⁺.