The Preparations Of Gallium Oxides And Their Physical Properties Under High Pressure

Gallium oxides with spinel structure have become the potential materials for microwave devices such as resonators and oscillators for satellite communication,positioning systems and wireless local area networks due to their good thermal stability,low dielectric constant and high quality factor.The research on their structure and performance is one of the hot spots at present.High pressure technique can be used to discovery new multifunctional materials with novel properties.In this thesis,AB₂O₄?A=Mg,Ca;B=Ga?have been synthesized successively by solid state and HTHP?high temperature and high pressure?method.The characterizations including X-ray diffraction?XRD?,transmission electron microscope?TEM?,scanning electron microscope?SEM?and Raman spectra measurements have been performed.The studies of crystal structure and lattice vibrations under high pressures have been carried out by using synchrotron radiation X-ray diffraction and Raman spectroscopy.The following results have been obtained:?1?Both solid state and HTHP methods were used to prepare spinel MgGa₂O₄.SEM showed that the synthetic pressure had strong influence on its morphology.?2?The diamond anvil cell?DAC?was used to investigate the structure stability of MgGa₂O₄ under high pressure.The results showed that no structural phase transition was observed within the pressure range of 0-38.6 GPa,and the bulk modulus B₀ was obtained to be 235.7 GPa by fitting the equation of state,which was the largest value among the spinel oxides.?3?YbMgGaO₄ and CaGa₂O₄ with new cubic structure and YbMgInO₄ with trigonal structure were successfully prepared by solid state reaction and HTHP methods.?4?Raman modes for the cubic structure CaGa₂O₄ had been assigned as:342.0 cm^-1for E_g mode,662.0 cm^-1 for T_2g?1?mode,765 cm^-1 for A_1g mode.These vibration modes were involving with the oxygens of AO₄ in tetrahedral symmetry as bending vibration,antisymmetric stretching vibration or asymmetric bending vibration and symmetric stretching vibration.In addition,254.0 cm^-1 corresponded to the vibration mode of T_2g?2?,which was related to the translation mode of BO₆ octahedron with respect to A cation.For YbMgGaO₄,the E_g modes were located at 340.5 cm^-1,617.1cm^-1,699.0 cm^-1,and A_1g modes were at 393.7 cm^-1,501.2 cm^-1,and 754.4 cm^-1.In A_1g mode,two atoms vibrated in opposite directions along the c-axis.In E_g mode,they vibrated in opposite directions paralleled to Yb-O and Mg-O/Ga-O planes.While the lowest energy mode E_1g corresponded to the displacement in the ab plane,where every two-layered atomic layer moves in the opposite direction.