Pressure-induced Structural Evaluations And Electronic Transition In Zn_0.2Mg_0.8Fe₂O₄ And Co_0.5Mg_0.5Cr₂O₄ Spinels

Cr-based and Fe-based spinel oxides with unfilled d-orbital are very interesting due to their broad applications.These are usually found in the interior of our Earth.These facts make its high pressure studies essentially important to materials science,physics,geosciences,and crystallography.In this thesis,the changes of crystal structure and electronic properties of the spinals of Zn0.2Mg0.8Fe2O4 and Co0.5Mg0.5Cr2O4.under high pressures has been carried out,and the main results are as follows:In the first project,the effect of pressure on the electronic properties and crystal structure in a mixed spinel ferrite Zn0.2Mg0.8Fe2O4 was studied for the first time up to 48 GPa at room temperature using x-ray diffraction,Raman spectroscopy,and electrical transport measurements.The sample was cubic(spinel-type-Fd3m)at ambient pressure and underwent a pressure-induced structural transition to an orthorhombic phase(CaTi2O4-type-Bbmm)at 21 GPa.This structural transformation corresponded to a first-order phase transition that involved 7.5%molar volume shrinkage.The onset of the Mott insulator-metal transition(IMT)around 20 GPa was due to a spin crossover mechanism that lead to the Fe3+magnetic moment collapse.It was found that all the Raman modes disappeared at high pressures which supported metallization.Analysis of structural and electrical transport measurements showed a simultaneous volume collapse and sharp IMT within a narrow pressure range.The orthorhombic high-pressure phase was found to have a higher conductivity than the cubic phase.The pressure dependence of the conductivity supported the metallic behaviour of the high-pressure phase.In the second project,the effect of pressure on the structural,vibrational,and electronic properties in Mg doped Cr bearing spinel Co0.5Mg0.5Cr2O4 has been studied up to 55 GPa at room temperature using x-ray diffraction,Raman spectroscopy,electrical transport measurements and ab initio calculations.The ambient-pressure phase is cubic(spinel-type,Fd3m)and undergoes a pressure-induced structural transition to tetragonal phase(space group I4m2)around 28 GPa.This structural transformation corresponds to a first-order phase transition.The changes caused by the transition in the electronic and vibrational properties are discussed as well as the pressure dependence of the vibrational and transport properties in each phase.The behavior of Co0.5Mg0.5Cr2O4 is compared with that of other spinel-type oxides.We found the low-and high-pressure phases to be wide-band gap semiconductors.The tetragonal high pressure phase hinders the conduction behavior while Cubic Phase supported conductivity behavior with compression.No evidence of chemical decomposition or a semiconductor-metal transition is found in our studies.