Study On Thermal Expansion Properties Of Cs₂W₃O₁₀

Thermal expansion and cold contraction are the properties of most materials in our daily life,but they often affect the service life of materials and bring unnecessary losses.In order to solve this problem,the research of near zero expansion materials has become one of the hot spots in the field of functional materials.Compared with near zero thermal expansion composite materials,single-phase near zero-expansion materials have certain advantages in application.Single phase near zero expansion materials have better thermal shock resistance and mechanical properties,so it is of great significance to search for single phase near zero thermal expansion materials.Cs₂W₃O₁₀has excellent self-excited UV luminescence and photocatalytic properties.Systematic study of its thermal expansion properties will help the material to be widely used further.The thermal expansion property of Cs₂W₃O₁₀was studied in this paper.The main work is as follows:(1)The tetragonal sample Cs₂W₃O₁₀has been prepared by a solid state method.The cell parameters of the sample at room temperature are a=b=16.1040(1)(?),c=10.1899(1)(?),V=2642.64(3)(?)³by Rietveld structural refinement method.The crystal structure schematic obtained from the fractional coordinates of the atoms shows that corner-sharing WO₆octahedral form a frame structure,and Cs atoms fill in the pore positions of the frame.The apparent linear expansion coefficient of the sample has been measured using a dilatometer.It was found that the thermal expansion of the sample was negative in the temperature range of 300-385 K,and the linear expansion coefficient was-3.01×10^-6K^-1.The thermogravimetric curves of the sample were measured using a thermal analyzer.When the temperature increased from 300 K to 573 K,the weight loss was less than or equal to 0.4%,indicating that the decrease of the apparent expansion coefficient in the temperature range from 300to 385 K was not caused by water absorption.(2)The XRD patterns of the samples were measured by a variable temperature diffractometer at 150–573 K.The samples maintained a tetragonal structure in this temperature range,and no structural phase transition occurred.The parameters of the cell at different temperatures were obtained by structural refinement,and the intrinsic linear expansion coefficients of different temperatures were calculated as 0.94×10^-6K^-1(150–300 K),-0.31×10^-6K^-1(300–385 K)and 0.84×10^-6K^-1(385–573 K),respectively.The results of the intrinsic linear expansion coefficient show that the sample has a near-zero expansion characteristic in the temperature range of 150-573K.In the temperature range of 300–385 K,the sample shows a negative thermal expansion characteristic as measured by the apparent linear expansion coefficient.It can be seen from the variation curves of axes a and c with temperature.The change of expansion coefficient is mainly due to the negative thermal expansion of c axis in the temperature range of 300–385 K.According to the position information of each atom in the cell at different temperatures,the variations of W-O-W bond angles along the c-axis along different temperatures were obtained.It is found that the negative thermal expansion of the c-axis in the temperature range of 300–385 K should be the result of the superposition of the variations of the W-O-W bond angles along the c-axis.(3)The pressure-dependent Raman spectra and temperature-dependent Raman spectra of the samples were obtained by Raman spectrometer.When the pressure increases from 0 GPa to 9.00 GPa,the corresponding W-O-W bending Raman vibration mode with a wave number of 172 cm^-1has a red shift with the pressure,which indicates that the vibration die has a negative Gruneisen parameter.When the temperature increases from 123 K to 673 K,the two Raman vibration modes corresponding to WO₆octahedral libration with wave numbers of 35 cm^-1and 46 cm^-1shift blue with temperature,and the total anharmonic is positive.Combined with the changes of crystal structure with temperature,the libration of WO₆octahedron connected by common vertices and the W-O-W bending vibration should be the main reasons for the near zero expansion of Cs₂W₃O₁₀,and the low negative thermal expansion in the temperature range of 300-385 K may be related to the enhancement of these two kinds of vibration.The study of near zero expansion of Cs₂W₃O₁₀is beneficial to understand its thermal characteristics and promote the wider application of materials.Meanwhile,the exploration of the reason for its intrinsic near zero expansion also enriches the mechanism of near zero expansion of frame oxide materials.