Study On The Structure And Properties Of Curved Molecule C₁₈Te₃Br₆(Bu-O)₆ And Negative Thermal Expansion Material Sc₂W₃O₁₂ Under High-Pressure

Pressure is one of the three basic thermodynamic parameters independent of material composition and temperature.It can change the structure of materials.Some substances that could not be synthesized at atmospheric pressure were synthesized at high pressure.In this paper,curved molecule C18Te3Br6(Bu-O)6 and negative thermal expansion material Sc2W3O12 have been tested by high-pressure in-situ Raman spectroscopy,high-pressure synchrotron radiation XRD,high-pressure infrared absorption spectroscopy and high-pressure AC impedance spectroscopy by Diamond Anvil Cell(DAC).The properties of C18Te3Br6(Bu-O)6 and Sc2W3O12 under high pressure were systematically studied,including phase transformation,crystallization and piezopolymerization,and the properties of the materials under high pressure were analyzed.The surface molecule C18Te3Br6(Bu-O)6 was tested by high-pressure infrared absorption,high-pressure Raman scattering,high-pressure synchrotron radiation XRD and high-pressure AC impedance spectroscopy.It was found that the pressure-induced dehydro-Diels-Alder reaction of C18Te3Br6(Bu-O)6 took place at?3 GPa,and the decomposition products were C18Te3Br6(3-butenyl-O)6 and hydrogen.There is a structural correlation between C18Te3Br6(3butenyl-O)6 and reactants.At?15 GPa,the pressure induced polymerization of C18Te3Br6(BuO)6 occurs,and the intramolecular sp2 bond is transformed into the intermolecular sp3 bond(pressure-induced diamondization),and a good carrier channel is established,which rapidly reduces the resistance.In addition,a critical pressure of-45 GPa was found.When the maximum pressure is lower than 45 GPa,the transformation from sp2 to sp3 bond is not completely reversible,and all C-C bonds formed between adjacent molecules break,but some sp3 bonds on the molecular edge are preserved,forming curved fragments composed of several molecules.When the maximum pressure is higher than 45 GPa,graphitization occurs in the process of pressure relief of C18Te3Br6(Bu-O)6.All sp3 bonds on the molecular edge of C18Te3Br6(Bu-O)6 remain unchanged,and only a few sp3 bonds between molecular layers are broken,forming a low sp3 state.We performed high-pressure Raman scattering and high-pressure variable temperature AC impedance spectroscopy experiments on Sc2W3O12,respectively.The high-pressure Raman results show that Sc2W3O12 undergoes a structural phase change at?0.4 GPa,and its structure changes from orthogonal to monoclinic phase.At?2.3 GPa,another phase transition was experienced,and the Sc2W3O12 structure began to become disordered when the pressure continued to be pressurized to?6.1 GPa,and when the pressure reached?16.3 GPa,Sc2W3O12 had been amorphized.According to the study of the variable temperature high-pressure AC impedance spectrum of Sc2W3O12,the temperature can change the conductive mechanism of Sc2W3O12,at 50?,Sc2W3O12 has undergone changes from electron conduction to ions,and with the increase of temperature,the more obvious the change in conductive mechanism,and with the change of pressure,the resistance of Sc2W3O12 also changes due to phase change and other reasons.Although both pressure and temperature can change the electrical transmission properties of Sc2W3O12,the effect of temperature is more significant.It is hoped that the Sc2W3O12 material with near zero thermal expansion can be found by controlling temperature and pressure.