The Electrical Transport Properties Of Metal Oxides Under High Pressure

Metal oxide materials are important functional materials with excellent physical and chemical properties,which are widely considered applicable in electrochemical energy storage devices,optoelectronic devices,gas sensors,and etc.The research on the structure,electrical properties and transport mechanism of typical metal oxides is of great practical significance to enrich the basic research of metal oxides,search for new materials with better properties,and further promote the application of metal oxides.In this dissertation,through combining the high-pressure synchrotron X-ray diffraction,Raman spectra,impedance spectroscopy,ultraviolet-visible absorption spectra measurements and simple first-principle calculations,we have investigated systematically the structure and electrical transport properties of metal oxides?-GeO₂,SnO₂,TiO₂,Mn₃O₄ based on the diamond anvil cell,and a series of creative research results are achieved.The summaries are as follows:1.The structure evolution of?-GeO₂ upon compression was studied by the in-situ Raman spectra and the first-principles calculations.In addition,the electrical transport behaviors of?-GeO₂ under high pressure were investigated by in-situ alternating current impedance spectra measurements.At 5 GPa,the coordination number of Ge in?-GeO₂ changed from 4 to 6 under pressure,and then sample became amorphous.And amorphous state can be retained to ambient condition at reduced pressure.The sudden change of ionic resistance at 4.6 GPa can be attributed to the change of Ge coordination number,during which many nonbonded oxygen atoms appeared near the GeO₄tetrahedra,subsequently leading to an increase of the oxygen ion concentration.In addition,the dielectric properties of GeO₂ can be effectively modified with compression,the frequency response range of dielectric loss is widened,and the dielectric loss in the low frequency region is significantly reduced.These results not only provide a valuable approach to further improve the dielectric properties of amorphous Ge oxides,but also lay the physical basis for exploring the application of Ge oxides in optoelectronic devices.2.The structure and electrical properties of bulk SnO₂ under heating and non-hydrostatic compression were studied by in-situ synchrotron X-ray diffraction,alternating impedance spectra and variable temperature resistivity measurements.Compared to previous XRD experiments?21 GPa,room temperature?,it is discovered that cubic fluorite SnO₂ can be achieved and retained to ambient condition at reduced pressure,12.0 GPa,after heating to 500 K by combining the uniaxial compression,slow heating and cooling.The fluorite phase of SnO₂ has about 10 times higher conductivity than its rutile phase and can be preserved to ambient after the sample was heated to 500K at 12.0 GPa.In this work,it is an important discovery to preserve SnO₂'s fluorite phase to ambient condition through uniaxial compression,slow heating and cooling,which provides a new perspective for capturing high pressure phase with better properties.3.The electrical transport behavior of anatase TiO₂ with an average particle size of 11 nm and 62 nm under high pressure was investigated with impedance spectra,UV absorption spectra and high-resolution transmission electron microscopy.In addition,combining with high-pressure Raman measurements,we characterized the effect of grain boundaries on the electrical transport and structure properties under high pressure.The electrical measurements revealed that the difference of electrical transport mechanisms of anatase TiO₂ with different size under compression could be attributed to the effect of grain boundary.The Raman measurements showed two different pressure induced phase transition sequence for the two different nano-size samples.The atomic arrangement at the grain boundary is in the transition state from one direction to another,where break and formation of chemical bonds need more energy than in the grain,which inhibits the formation of new phase.Therefore,the size-dependent grain boundary effect can bring some non-negligible impact on the phase transition sequence of anatase TiO₂.Our research reveals the mechanism of the size effect on the electrical properties of nano anatase TiO₂ under high pressure,and provides a new perspective for the study of the size effect of nano materials in high pressure phase transformation.4.The electrical transport properties of Mn₃O₄ under high pressure were studied by alternating current impedance spectra and UV-vis absorption spectra.The impedance spectra results indicate that below 4 GPa,Mn₃O₄ always presents pure ionic conducting upon compression.However,above 4 GPa,the conduction mechanism changes from ionic to the coexistence of ionic electronic,which can be attributed to the decrease of band gap under pressure.Above 12.2 GPa,the conduction mechanism becomes pure electron conduction,which is caused by pressure-induced tetragonal spinel to orthorhombic marokite-type structure transition.Meanwhile,the high-pressure UV-vis absorption experiment results also show that the optical band gap of Mn₃O₄ decreases suddenly in the pressure range from 12.1 to 20 GPa.It is indicated that the improvement of conductivity of sample above 12.2 GPa is due to the decrease of band gap.The pressure-induced disappearance of ionic conduction,the narrowing of band gap and the improvement of conductivity in Mn₃O₄ have opened up a new way to improve the electrochemical performance of Mn₃O₄ based electrode materials.