The Thermal Expansion Properties And Raman Spectroscopy Of AMgMo₃O₁₂?A=Zr,Hf?

As we know,most materials have the property of"thermal expansion"Thermal expansion and contraction of materials is a common problem in many important areas,such as optics,machinery and electronics,aerospace,communications,etc.they all face with it.When temperature increases,the expansion coefficient of the material does not match resulting in micro cracks,because the instrument is made from different materials.This will affect the performance of the instrument.Now scientists have discovered a new type of material,we call it the negative thermal expansion material which has the nature of"cold thermal expansion".Research and development of thermal expansion of the material or near zero thermal expansion will solve the problem in instrument that thermal expansion coefficient of the material does not match.We can also enhance the thermal shock resistance of the material and prolong its life.In recent years,the study of expansive material system is increasingly perfect,the performance of a single material may not meet our needs,so it is necessary to design a negative expansion material that meets our needs through theory and experiment.First of all,it has reported that ABM₃O₁₂ was discovered based on A₂M₃O₁₂,by substituting with the valence atom.the NTE of ZrMgMo₃O₁₂ is-3.7×10^-6 K^-1 and HfMgMo₃O₁₂ is 1.02×10^-6 K^-1,their phase transition point are blow the room temperature without hygroscopicity.We designed this experiment for the purpose of obtaining a material with a slight variation of expansion coefficient as the temperature goes up.We design the chemical formula for Zr_xHf_1-xMgMo₃O₁₂?x=0-1?oxide ceramics,through the equivalent substitution of Zr⁴⁺and Hf⁴⁺to get a material with the expansion coefficient is a near zero expansion material between ZrMgMo₃O₁₂ and HfMgMo₃O₁₂ in a larger temperature range.In this experiment,the four proportions of material,Zr_0.3Hf_0.7MgMo₃O₁₂,Zr_0.5Hf_0.5MgMo₃O₁₂,Zr_0.7Hf_0.3MgMo₃O₁₂ and Zr_0.8Hf_0.2MgMo₃O₁₂ were prepared respectively,and we measured their expansion properties,crystal structure,space group,and phase transition temperature area.The following are the main results and innovations of this paper.1.Structure analysis was performed by the Le Bail fit of the 297 K XRD pattern.The fitting result shows that there are all adopt orthorhombic structure with space group Pnma?No.62?.Take the results of Zr_0.5Hf_0.5MgMo₃O₁₂ for example.The lattice constants of Zr_0.5Hf_0.5MgMo₃O₁₂ are a=9.5768?,b=9.4897?,and c=13.1818?,respectively.And it with acceptable values of Rexp=3.83%,Rwp=7.61%,and Rp=5.78%.The ratio of Rwp to Rexp is 1.32,so the data is more reliable.2.Temperature-dependent X-ray diffraction pattern and thermal expansion apparatus indicate that the expansion properties of this series of ZrxHf1-xMgMo3O12?x=0.3,0.5,0.7 and 0.8?are near zero?273 K-673 K?.The change of the expansion coefficient of Zr_xHf_1-xMgMo₃O₁₂?x=0.3,0.5,0.7 and 0.8?was attributed to the electronegative difference between Hf⁴⁺and Zr⁴⁺.With the increase of Zr⁴⁺content,the distortion of octahedron and the rigidity of MoO₄ tetrahedron were weakened.Compared with HfMgMo₃O₁₂,the anisotropy of Zr_0.5Hf_0.5MgMo₃O₁₂ is smaller and negative expansion performance is enhanced,it has achieved near zero expansion.3.Raman spectra of Zr_xHf_1-xMgMo₃O₁₂ with x=0.3,0.5,0.7 and 0.8 at different temperatures shows that the characteristic Raman modes at 1005 cm^-1 of monoclinic phase are disappeared at 153 K-148K,148 K-153 K,138 K-143 K and 128 K-133 K,respectively.This phenomenon indicated that the material had changed from monoclinic phase to orthorhombic phase,and the phase transition temperature range is changed regularly between ZrMgMo₃O₁₂ and HfMgMo₃O₁₂.The appearance of the lower phonon modes at 90 cm^-1,158 cm^-11 and 183 cm^-1 of Zr_xHf_1-xMgMo₃O₁₂?x=0.3,0.5,0.7 and 0.8?are appearance when temperature increasing at their phase transition temperature range,respectively,leading to the negative thermal expansion.