Phase Transition And Thermal Expansion Controlling Of Complex Oxides With Framework Structure

The negative thermal expansion （NTE） materials of the open-framework structure is a very large family, including SCF₃, A₂M₃O₁₂ （A=trivalent metal cation and M = Mo or W）, ZrM₂O₈ （M=W or Mo）, Metal-Organic Frameworks （MOF）, and so on. It is significant to investigate their properties both for fundamental studies and practical applications. In the present study, we investigate the materials with the formula of A₂W₃O₁₂ and ZrM₂O₈. So far, a lot of investigations on A₂W₃O₁₂ and ZrM₂O₈ have been carried out. But these studies focus on the chemical substitutions to adjust the phase transition and thermal expansion. In this paper, we adjust and explore the thermal expansion of A₂W₃O₁₂ and ZrM₂O₈ by two ways:the accommodation of small molecules （NH₃） into the void of the framework structure and using the technology of high temperature quenching.Through ammoniation, the small molecule NH₃ is successfully inserted into the framework structure of ZrW₂O₈ and forms a weak coordinate bond with W2. This insertion makes ZrW₂O₈ more stable and rigid, which increases the phase transition temperature about 50℃, and decreases the coefficient of thermal expansion （CTE） from -7.8×10^-6℃^-1 to -2.1×10^-6℃^-1. The possible reasons for these changes are that the insertion of NH₃ decreases the rotation space and attenuates the dynamic rocking motions of polyhedrons. Simultaneously, the ammoniation can prevent ZrW₂O₈ from hydration. In addition, the small molecule of NH₃ can be also successfully inserted into ZrWMoOg, ZrMo₂O₈ and Y₂W₃O₁₂ by ammoniation. And the ammoniated ZrWMoO₈ and ZrMo₂O₈ have a similar behavior with the ammoniated ZrW₂O₈. While ammoniated Y₂W₃O₁₂ has a quite different structure from Y₂W₃O₁₂.Dy₂W₃O₁₂ presents monoclinic phase without NTE property at low temperature, and transforms to orthorhombic phase at high temperature, which shows NTE behavior. The phase transition temperature was determined at 996℃。 Orthorhombic Dy₂W₃O₁₂ can be retained to low temperature by quenching. The quenched sample is stabilized below 500℃ and shows NTE behavior (-2.6×10^-5℃^-1 in the temperature range of 150～500℃. This value is the largest coefficient of negative thermal expansion in A₂W₃O₁₂ family. In addition, the photoluminescence and magnetic properties of quenched sample were also investigated.The phase formation of HO₂W₃O₁₂ is a datable issue, and no detailed analysis was reported. In the present study, we synthesized HO₂W₃O₁₂ with different treatments to probe the relationship between phase formation and synthesized conditions. It is found that the thermodynamic stable phase of low temperature is monoclinic, and transforms to orthorhombic at high temperature. The temperature of phase transition was determined at 908℃. The phase transition rate is slow. And there is a close relationship between the phase formation and the cooling rate. The single phase of orthorhombic Ho₂W₃O₁₂ can be retained at low temperature even at the cooling rate of 10℃/min. Moreover, the stability and the thermal expansion of orthorhombic Ho₂W₃O₁₂ were also investigated. The orthorhombic phase can be stabilized by 600℃, and show negative thermal expansion （NTE） along three axes with the overall volume thermal contraction of-2.1×10^-5℃^-1.In the solid solution of Gd_xTm_2-xW₃O₁₂（x=0.5,1 and 1.5）, Gd_0.5Tm_1.5W₃O₁₂ is in orthorhombic phase. GdTmW₃O₁₂ and Gd_1.5Tm_0.5WsO₁₂ are in monoclinic phase at room temperature, and transform to orthorhombic phase at high temperature. Through high temperature quenching, orthorhombic GdTmW₃O₁₂ and Gd_1.5Tm_0.5W₃O₁₂ were remained in room temperature, which are more hygroscopic than Gd_0.5Tm_1.5W₃O₁₂. The thermal stability and thermal expansion properties of them were investigated. The coefficient of thermal expansion increases along with the increasing of Gd contents.Our work shows that it is significant to explore the properties of A₂W₃O₁₂ and ZrM₂O₈ by inserting small molecules （NH₃） into the void of the framework structure and by high temperature quenching, which are helpful for further research and potential applications.