The Structural And Electrical Propetries Study Of MNbO₃（M=Li, Na, K） And B₁₂As₂under High Pressure

The electrical properties and structures of materials are highly interelated underhgih pressures. Via the study of electrical properties of materials under high pressure,some special information about structure variation that cannot be detected by otherdetection techniques can always be found. For example, via the study of dielectricbehaviours in the aspects of dispersion relation, media absorption, dielectric loss anddielectric relaxation, structure informations such as the nature of the chemical bond,the rotation of the molecule, and ion vibration process as well as their evolutions canbe known under high pressures; via the study of electrical resistivity of polycrystalline samples, the scattering of charge carriers by grain boundaries can beanalyzed and the density of grain boundaries can be reflected; with the study ofdielectric relaxation and loss of grain boundaries, the effect of pressure on spacecharge layers can be known; in addition, through the abnormal magnetoresistanceeffect, the scattering of spin waves by grain boundaries can be understood. It issignificant therefore to study variant electrical properties of materials under highpressure for deeply understanding and discussing the properies of ion bonding,molecue polarization in structures.In this thesis, we systematically measured under high pressures the dielectricbehavious and the grain boundary transportation processes of MNbO₃（M=Li, Na, K）and B₁₂As₂, analyzed and explained these properties from microscopic structure pointof view, and found some new phenomena. Study results are listed as follow:1. Double-plate electrodes for in situ dielectric properties measurements under highpressures have been integrated on diamond anvil cell by thin-film deposition andphotolithography technique. Taking advantage of the fixed shape and position ofdouble-plate electrodes, we measured variant dielectric parameters of materialsunder high pressures accurately. In measurements we have taken the edge effectinto account, and therefore, using finite element analysis, analyzed quantitatively the influence of the electrode configuration on the measurements to ensure theaccuracy of results.2. Using high-pressure in situ impedance measurements, high-pressure X-raysynchrotron radiation and first-principles calculations, we studied the electricaltransport properties and structural phase transitions of MNbO₃（M=Li, Na, K）. Itis found that （1） for LiNbO3, ionic and electronic conductions coexist, and theionic conduction dominates the transport processes in the entire investigatedpressure range. The pressure-induced structural phase transition changes the grainboundary microstructures, resulting in corresponding change of transportparameters. In R3c phase, pressure increase makes the Li+ions diffusion easierbut the electronic transport through the grain boundaries difficult. In Pnma phase,the pressure increase continuously makes the Li+ions diffusion easier, but it haslittle effect on the electronic transport, which results from that the electronicconduction in grains is local type and does not change with pressure. In the R3cphase, the pressure increase makes the charge-discharge process difficult, whilemakes it much easier in the Pnma phase. The change of resistance with pressure iscaused by the bandgap changes in different phases, which causes the change of theelectron concentration. In the Pnma phase, electrons transfer from Nb⁵⁺ion to O^2-ion with increasing pressure, resulting in the strengthen of electron localizationaround O atoms, and thus leading to the decrease of the relative permittivity andthe difficulty of polarizaion of Nb-O dipoles. In the R3c phase, the charge transferbetween Nb⁵⁺and O^2-is not obvious and therefore the relative permittivity has nodistinct change with increasing pressure. The microstructures of grainboundariesreconstruct after phase transition and cannot return to their original state afterdecompression. After one pressure cycle, the dielectric loss in low frequency issignificantly reduced, and therefore, the dielectric performance of materials can beimproved by modulating the grain boundary microstructures.（2） NaNbO₃has aPbcm structure at ambient conditions. High-pressure X-ray diffraction indicatesthat there are two structural phase transitions at around7.6GPa and15.5GPa.The HPI phase is preliminary identified as Pna21structure and the HPII phase isunknown. The ionic conduction dominates the transport process in NaNbO₃. Forthe high-pressure ions diffusing properties, in the Pbcm phase, the pressureincrease makes the Na⁺ions diffusion difficult, while in the Pna21and HPIIphases, pressure causes respectively a rapid and a slow increase in the diffusion coefficient. The different modulation of diffusion coefficient by pressure iscaused by that Na⁺ion has different diffusing path lengths in different diffusionlayers. For the high-pressure ions conductivity, in the Pbcm phase, the ionicmigration channel is significantly compressed with increasing pressure, in whichthe size of some channels becomes comparable with or smaller than that of Na⁺ions, thus increasing resistance to or blocking the Na⁺ions migration and leadingto a rapid decrease of the ionic conductivity. In the Pna21phase, the pressureincrease makes the ionic channel larger and therefore the ionic conductivityincreases. In the HPII phase, most of the ionic channels are compressed slowlywith the pressure increase and accordingly the ionic conductivity decreases. Forthe high-pressure relative permitivity, in the Pbcm phase, it decreases rapidly withthe pressure increase, resulting from the rapid increase of activation energy andthe enhancement of dipole resonance damping in the ionic polarization increases.However in the Pna21and HPII phases, the relative permitivities still decreaseslowly with the pressure, even while the dipole resonance damping decreases.This phenomenon can be explained as: in the Pna21and HPII phases, besides theionic polarization, there has the space charge polarization of interface layer. Thepressure not only impacts the ionic migration channels, but also weakens theconductivity and the polarization of the interface layer, which leads to a slowdecline of relative permittivities. In the entire investigated pressure range, therelative permittivity continously changes with increasing pressure.（3） KNbO₃hasnot ionic conduction, and primarily, the electron transportation within thegrainboundaries has no contribution to the whole conductions. The decrease of theresistance of orthohombic and tetragonal phases of KNbO₃is caused by thenarrowing of bandgap and the increase of charge carrier concentration, while theincrease of cubic phase resistance results from the widening of bandgap and thedecrease of charge carrier concentration. In the orthorhombic and tetragonalphases, the pressure increase weakens the Nb-O dipole resonance dampling, andtherefore the relative movement between adjacent Nb and O atoms makes valenceelectrons more polarized and localized, in which electronic localization can affectthe orbital hybridizaion of Nb and O atoms and generate bigger Nb-O dipolemovement, leading to easier charge-discharge process. Opposite phenomenon canbe found in the cubic phase. In the orthorhombic and tetragonal phases the chargedistribution transfer happens from O^2-ion to Nb⁵⁺ion, which results in an easier Nb-O dipole polarization; in the cubic phase, the direction of electron transferringis opposite, namely from Nb⁵⁺ion to O^2-ion, leading to stronger electroniclocalization around O atoms and more difficult Nb-O dipole polarizaion, sorelative permittivity increases with increasing pressure in the orthorhombic andtetragonal phases, but dereases in the cubic phase.3. Using the high-pressure impedance measurements, the grain boundary effect ofB₁₂As₂on the electrical transport behavior under high pressure has been studied.The results show that the grain resistance and the grain boundary resistancedecrease with increasing pressure and the contribution of the grain boundaryresistance to total resistance decreases after16.8GPa. Using the diffusion theorymodel, the variation of space charge potential with pressure is obtained. When thepressure is greater than20.7GPa, the space charge potential becomes negative,indicating that the grain resistivity is more than twice of the grain boundaryresistivity. With increasing pressure, the decrease of the contribution of grainboundary resistance can be explained as follow: the defect energy levels of thegrain boundaries （for instance, the vacancy and the entangled state energy levels）,may capture the holes from the grains, leading to a carrier depletion in the grainsand resulting in carrier accumulation in the grain boundaries.