Study On The Dielectric Phenomena Related To Oxygen-vacancy-ordering

Oxygen vacancies,also called F or color centers due to their specific optical absorption,are the ubiquitous point defects in oxides,which are known to be the culprits for various properties of oxides.For instance,with increasing the concentration of oxygen vacancies,SrTiO3 can change from a semiconductor to a metal,which becomes a superconductor at low enough temperatures.Oxygen vacancies play a decisive role in determining the critical temperature of high-temperature superconducting cuprates,magnetic properties of semiconducting oxides,catalysis behavior of mixed conductors,and so on.At high enough temperatures,the hopping motion of oxygen vacancies make significant contribution to ac conductivity,and therefore,dominate the high-temperature dielectric properties of oxides.One of the most interesting issues for oxygen vacancies is that the vacancies possess an inherent inhomogeneous distribution leading to self-ordering state in oxides.This phenomenon known as phase separation or stripe phase has attracted considerable interest in high temperature superconducting cuprates and colossal magnetoresistive manganites.In ferroelectric materials,the self-ordering of oxygen vacancies forming linear or planar clusters was proposed to be held responsible for fatigue and long-term degradation.It was reported that charge ordering can lead to colossal dielectric behavior or relaxor-like dielectric anomaly.Our experimental conclusions can be a good way to describe the ordering processes of oxygen vacancies in most oxides.In our work,the dielectric properties of TiO2,MgO,TeO2 and LiNbO3 single crystals were studied,and the following results were obtained:(1)The dielectric properties of TiO2 were studied in a wide temperature range(room temperature-700 0C)and wide frequency range(100 Hz-1 MHz).Detail dielectric measurements in the frequency domain reveal three temperature regions:In region ?(T? 200?),the electrons created by oxygen vacancies dominate the dielectric properties of TiO2 crystal causing the relaxation 1 due to electrons hopping between Ti4+ and Ti3+ ions.In region II(200 ?<T<500 ?),oxygen-vacancy clusters are the main relaxing entity giving rise to the phase transition.In region III(500 ? ?T),isolated oxygen vacancies becomes the main carriers.Their hoping motion between spatially fluctuating lattice potentials yields the relaxation 2.In addition,the Laue diffraction method confirms the presence of an oxygen vacancy ordering process in the temperature region II.First principles density functional theory(DFT)calculations reveal that the oxygen-vacancy-ordering is energetically outmatches the oxygen-vacancy isolated state and the oxygen-vacancy cluster favors forming a linear chain along[001]direction.(2)Dielectric properties of MgO single crystals were investigated in the temperature range of 300-1050 K and frequency range of 300-106 Hz.The sample exhibits intrinsic dielectric response with a dielectric constant of?9.5 below 400 K and two oxygen-vacancy-related phase transitions at the temperature around 800 K.Impedance analysis revealed the first phase transition is accompanied by a an electronic phase transition of metal-insulator transition(MIT).These results can be well understood based on the scenario of oxygen-vacancy-ordering process.The first and second phase transitions were argued to be related to oxygen vacancies changing from individual to collective and then returning to individual behaviors,respectively.(3)In the temperature range of room temperature to 1073 K and wide frequency range(100 Hz-2 MHz),only one dielectric anomaly is found in the TeO2 single crystal.An anomaly at?745 K shows a phase transition behavior.Oxygen vacancy distribution mechanism is considered to be the cause of this abnormality.The anomaly at 745 K indicates that the transition behavior is associated with a change in the oxygen vacancy distribution from a dynamic disordered state to a dynamic ordered state.As the temperature increases,the final state becomes dynamic disordered.(4)In all,only in the GLN and BLN,dielectric anomalies were found in the temperature range from room temperature to 1073 K and wide frequency range(100 Hz-2 MHz).Abnormalies at?607 K and?600 K are shown as phase transition behavior.It shows that the oxygen vacancy concentration will affect the phase transition,the concentration is too low there is no way to forming a phase transition.It is also found that the temperature of the phase transition is shifted to the low temperature when the electric field is applied.Indicating that the applied electric field can regulate the ordering process.