The Negative Thermal Expansion Property And Mechanism In The Zn₂V_2-xP_xO₇ And Zn_2-xCu_xP₂O₇ Compounds

The macroscopic volume change of most materials in nature is consistent with the change trend of temperature,that is,thermal expansion-cold contraction.But there are also some materials showing abnormal phenomenon-the macroscopic volume change and the temperature change trend are opposite,that is,thermal contraction-cold expansion.Zr W₂O₈is the representative of this king of abnormal substance.Duo to the emergence of negative thermal expansion material Zr W₂O₈,refreshed the traditional concept that materials only expansion with heat and contract with cold.After that materials with new molecular formulas have been emerging continuously,forming a series of systems.Although these materials have their own characteristics-provide new ideas for solving the problems of thermal stress and thermal deformation,they also have their own shortcomings,such as phase transition,narrow negative expansion temperature range or inability to cover room temperature,water absorption,stability difference and so on,which affect its practical application.So,improving the thermal properties of this type of materials have great value to promote its actual production.Zn₂V₂O₇and Zn₂P₂O₇exhibit negative expansion in the high temperature region,and their applications are greatly restricted.In this work,two negative thermal expansion materials,Zn₂V_2-xP_xO₇and Zn_2-xCu_xP₂O₇,were prepared by partial replacement of Zn₂V₂O₇and Zn₂P₂O₇by a partial ion substitution method,which realized the negative thermal expansion temperature range covering room temperature.The main research results and innovations are as follows:（1）Research on negative thermal expansion performance of Zn₂V_2-xP_xO₇.The melting point of Zn₂V₂O₇is 1163 K,and it isα-Zn₂V₂O₇phase at low temperature.When the temperature exceeds 883 K,it transforms intoβ-Zn₂V₂O₇phase.Theα-Zn₂V₂O₇phase has negative thermal expansion performance,but its negative expansion temperature range is 573-873 K,which is much higher than room temperature,which severely limits its application.In this work,a series of samples of Zn₂V_2-xP_xO₇（x=0,0.1,0.2,0.3,0.4,0.5）were prepared by partially replacing V⁵⁺with P⁵⁺.Found from the experimental results that the thermal expansion coefficient of Zn₂V_1.7P_0.3O₇is-4.47×10^-6K^-1（RT-673 K）,which greatly optimizes the negative thermal expansion characteristics ofα-Zn₂V₂O₇.Analysis suggests that the heat shrinkage mechanism of Zn₂V_1.7P_0.3O₇is:Lateral vibration of the O₃atoms intensifies with increased temperature（RT-673 K）,causing angles between Zn-O_3（long）-V and Zn-O_3（short）-V to decrease gradually.Thus,the interlayer spacing between the cells is gradually compressed.At the same time[Zn O₅]and[VO₄]polyhedrons undergo coupling twist and rotation changes,which intensify the coupling effect between them.And the influence of lattice distortion has led to the reduction of the volume of[Zn O₅]polyhedrons.The above combined action caused the volume of Zn₂V_1.7P_0.3O₇shrinks.（2）Research on negative thermal expansion performance of Zn_2-xCu_xP₂O₇.The melting point of Zn₂P₂O₇is 1288 K,and it isα-Zn₂P₂O₇phase at low temperature.When the temperature exceeds 403 K,it transforms intoβ-Zn₂P₂O₇phase.Theα-Zn₂P₂O₇phase has negative thermal expansion performance,but its negative expansion temperature range is 353-403 K,which is much higher than room temperature,which severely limits its application.In this work,a series of samples of Zn_2-xCu_xP₂O₇（x=0,0.1,0.2,0.25,0.3）were prepared by partially replacing Cu²⁺with Zn²⁺.Found from the experimental results that the expansion coefficient of Zn_1.75Cu_0.25P₂O₇is-21.4×10^-6K^-1（173-373 K）,which optimizes the negative thermal expansion characteristics ofα-Zn₂P₂O₇.Analysis suggests that the heat shrinkage mechanism of Zn_1.75Cu_0.25P₂O₇is:Zn_1.75Cu_0.25P₂O₇belongs to monoclinic structure.Two[PO₄]tetrahedrons share a bridge O₁atom to form a[P₂O₇]structural unit with Cs staggered symmetry.The anion[P₂O₇]group is centered on its mirror image plane and the bond angle between P-O-P is linear.The cations on the biaxial axis exist in the form of irregular[Zn O₆]octahedrons and share three edges with adjacent cations.Lateral vibration of the O₃atoms intensifies with increased temperature,causing angles between Zn（Cu）-O_3（long）-P and Zn（Cu）-O₃-Zn（Cu）to decrease gradually.At the same time[Zn O₅]and[VO₄]polyhedrons undergo coupling twist and rotation changes,which intensify the coupling effect between them.In addition,there is the phase transition effect of Zn_1.75Cu_0.25P₂O₇.The above combined effect caused the volume of Zn_1.75Cu_0.25P₂O₇to gradually shrink with the increase of temperature in173-373 K.