Structural Phase Transitions And Electrical Transport Properties Of CoWO₄ And ZnMn₂O₄ Under High Pressure

Both types of ABO₄ and AB₂O₄ compounds are important functional materials,which are widely used in industrial production and daily life.Pressure can not only induce the structure transformations of material but also change their properties and functions,so it is of great meaningful to study the properties of the both materials under high pressure.In this thesis,CoWO₄ and ZnMn₂O₄ are taken as the research objects to study the structure and electrical transport properties under compression,the specific experiments and analysis results are listed as follows:1.Pressure-induced structural phase transitions of CoWO₄The in situ Raman spectra of CoWO₄ under different pressures have been obtained by high pressure in situ Raman spectroscopy.There are 18 kinds of Raman vibrational modes in CoWO₄.The Raman spectra at atmospheric pressure were fitted by Lorentz,and the peak position and half peak width of each vibration mode of CoWO₄ were obtained under atmospheric pressure.With the increase of pressure,all Raman peaks decrease gradually and move toward the higher wavenumbers,which indicate a blue shift.When the pressure reach to 24.2 GPa,a new Raman peak appeares at 110 cm^-1.The new peak gradually moves to the direction of high wave number with the increase of pressure.With the continuous increase of pressure,the intensity of the newly emerged Raman peak gradually increased.The Lorentz fitting process was performed on the high-pressure Raman data in the Origin software,and the shifted peak position and pressure curve of the CoWO₄ Raman peak were obtained.We can also observe that the frequency shift of each vibration mode changed at 24.2 GPa in the diagram of each vibration frequency.The change in the Raman spectrum at 24.2 GPa may be due to the phase transition of the pressure-induced structure of CoWO₄.By high pressure in situ synchrotron radiation X-ray diffraction experiments,we have observed that all diffraction peaks of the sample move toward high angle with the increase of pressure.When the pressure increases to 26.8 GPa,two new diffraction peaks are observed,and with the increase of pressure,and the intensity of the two new diffraction peaks increase gradually.Combined with high pressure in situ Raman experiments,it can be determined that CoWO₄ undergoes a pressure-induced structural phase transition at 26.8 GPa.Both the atmospheric phase and the high pressure phase coexist to 45.0 GPa.When the pressure was released to atmospheric pressure,the diffraction pattern of CoWO₄ was restored to the structure of the initial phase,which means the phase transition of high-pressure structure of CoWO₄ is reversible.2.Structural phase transition and electrical transport properties of ZnMn₂O₄under high pressureThrough high pressure in situ Raman spectroscopy experiments,we have observed that all Raman peaks of ZnMn₂O₄ shows a blue shift with the increase of pressure.When the pressure reach 14.8 GPa,a new peak appears at 598 cm^-1.In order to obtain the relationship between the vibration frequency and the pressure of each vibration mode,The Lorentz fitting was performed on experimental data of high-pressure Raman spectroscopy,and the curve of Raman peak frequency shift under pressure was obtained.From the fitting results,we have observed that at the both two pressure points of 14.8 GPa and 23.4 GPa,the slope of frequency shift of peak positions change significantly.The change of Raman peaks at 14.8 GPa and 23.4GPa can be attributed to the pressure-induced phase transformation of the ZnMn₂O₄.By high pressure in situ synchrotron radiation X-ray diffraction experiments,we also observed that all diffraction peaks of ZnMn₂O₄ decrease with the increase of pressure.When the pressure reaches 13.7 GPa,two new diffraction peaks appear,the d value of the diffraction peak changes significantly at 24.1 GPa.Combining with high pressure in situ Raman experiments,it can be determined that the both changes of the sample at 13.7 GPa and 24.1 GPa are due to the pressure-induced phase transitions of the ZnMn₂O₄.From the high pressure in situ AC impedance spectroscopy experiments,we obserse that ZnMn₂O₄ has two semicircular arcs at low pressure and only one semicircular arc at high pressure,respectively.The pressure dependent resistance of ZnMn₂O₄ is obtained after the data processed by Z-view fitting.It was found that both grain resistance and grain boundary resistance decrease with the increase of pressure.At the pressure below 13.2 GPa,the grain and grain boundary resistance coexist,and the contribution of grain resistance to the total resistance was greater than that of grain boundary resistance;At the pressure above 13.2 GPa,the grain boundary effect disappears,only the grain effect can be detected.When the pressures reach 13.2 GPa and 21.5 GPa,the change rate of resistance vary obviously.Combining with the both results from the high pressure in situ Raman and synchrotron radiation X-ray diffraction experiments,the changes of AC impedance spectroscopy at 13.2 GPa and21.5 GPa are attributed to the pressure-induced phase transitions of the Zn Mn₂O₄.