Regulation And Mechanism Of Negative Thermal Expansion In Mg₂P₂O₇ And Cu_2-xZn_xVPO₇

Thermal expansion is an inherent basic physical property of materials.Modern high-precision devices have closely related to the thermal expansion performance of materials.Therefore,the realization of controllable thermal expansion is an important and urgently scientific problem.The emergence of negative thermal expansion（NTE）materials makes it possible.However,there are few kinds and quantities of materials with giant NTE coefficient,and the NTE temperature zone is narrow,which limited the application.Therefore,this paper takes Mg₂P₂O₇ and Cu₂VPO₇ as the research object,adopts a chemical substitution method to regulate the thermal expansion coefficient and temperature region,and clarifies the thermal expansion regulation mechanism.The main work contents are as follows:The giant NTE property of Mg₂P₂O₇ in a narrow temperature region was reported.Its linear coefficient of thermal expansion（α_l）is-57.6×10^-6 K^-1 from 337 K to 351 K.And the NTE properties was tailored by equivalent substitution method to add V or Cu atoms,respectively.The NTE coefficient of Mg₂P_2-xV_xO₇ was enhanced from-57.6×10^-6 K^-1（Mg₂P₂O₇）to-127.5×10^-6 K^-1(Mg₂P_1.9V_0.1O₇),and the NTE region was regulated to room temperature.The NTE temperature zone of Mg_2-xCu_xP₂O₇ can be broadened from 14 K（Mg₂P₂O₇）to 100 K(Mg_1.9Cu_0.1P₂O₇).The joint study of Raman scattering and X-ray diffraction indicated that Mg_1.9Cu_0.1P₂O₇ has two phases in its NTE temperature zone,and its NTE can be induced only when the volume shrinkage caused by the transformation from the largeαto the smallβ-phase can compensate for the positive thermal expansion（PTE）of the lattice.The role of the Cu atom mitigated the transition rate fromα-toβ-phase and prolonged the temperature range of two-phase coexistence,which broadened the NTE temperature zones.This work provides some insights to optimize the NTE property in most phase transition materials.Replacing part of Cu with Zn atoms enhanced the NTE property of Cu₂VPO₇-based ceramic materials.The linear coefficient of thermal expansion of Cu_2-xZn_xVPO₇increased from-16.6×10^-6 K^-1（Cu₂VPO₇）to-24.9×10^-6 K^-1(Cu_1.8Zn_0.2VPO₇)at RT～600 K.The X-ray diffraction shows that the monoclinic structure（space group C2/c）can be maintained when the substitution amount of Zn atoms was less than 20%.Crystal structure refinement indicated that the addition of Zn atom reduced the unit cell volume.The results of temperature-dependent Raman scattering indicated that with the increase of temperature,the transverse thermal vibration of oxygen atoms gets to enhance,then resulting to the NTE behavior.