【作者】 李娟；

【导师】 韩永昊；

【作者基本信息】 吉林大学 ， 凝聚态物理， 2022， 硕士

【摘要】 碘酸钾（KIO₃）是一种典型的钙钛矿型铁电材料,因其有较大的非线性光学系数和压电系数,可用于制造具有良好光学性能的非线性光学器件。作为独立于温度和化学组分的热力学参量,压力能够调整原子间距和原子间的成键方式,从而引发晶体结构相变和电子结构相变。相变后物质会表现出常压下不存在的新奇特性,从而拓展了材料的研究和应用领域。目前,KIO₃的实验研究多集中在性质随温度的变化关系上,在高压下开展的KIO₃性质研究还不甚全面。因此,本论文利用金刚石对顶砧（DAC）高压产生装置,结合原位交流阻抗谱测量技术与第一性原理计算方法,对KIO₃材料在高压下的能带结构、电子态密度、导电机制和介电性质展开了系统的研究,目的在于了解KIO₃材料在高压下的电输运行为。研究结果如下:1、常压X光衍射实验表明,本研究中的KIO₃材料的常压结构为三斜结构,空间群为P1。通过第一性原理计算,得到KIO₃在常压下带隙宽度为3.066 e V,价带的顶部平坦,导带的底部比较分散,因此导带的离域性更强。P1相的KIO₃材料在高压下带隙值逐渐减小,电子从价带跃迁到导带更容易,因此电子电阻也就越小。分析电子态密度得到价带的顶部和导带的底部分别来自O（2p）和I（2p）态,O和I原子的s轨道与p轨道之间存在很强的杂化。通过电子局域函数可知:O和I原子之间形成共价键,在压力为4 GPa时,O原子周围的电子被压缩,迁移至O和I原子的空隙位置,材料存在电子传导。2、通过高压原位交流阻抗谱测量,发现在压力小于5.6 GPa时,KIO₃材料的导电机制为离子-电子混合导电;在4.3～5.6 GPa之间时,交流阻抗谱低频区出现代表离子扩散过程的倾斜尾线,说明压力使得晶界微结构发生变化,⁺离子的扩散更加容易。压力在5.6～20.6 GPa时,KIO₃材料表现出离子传导机制,当压力增加到10.0 GPa时,阻抗谱上扬线的倾角几乎为0^°,⁺离子的扩散变得困难。3、利用等效电路对交流阻抗谱进行拟合,发现晶界与晶粒电阻在4.3 GPa和10.0 GPa处均发生了突变,对应着KIO₃的压致结构相变。在0～20.6 GPa范围内,晶粒电阻始终小于晶界电阻,晶界对材料的总电阻贡献较大。通过研究KIO₃相对介电常数、介电常数的实部‘、虚部‘‘和介电损耗tanδ与频率的关系,发现压力小于4.3 GPa时,相对介电常数在压力作用下逐渐增大,由O原子周围的电子局域性降低造成。在交变电场频率大于10 Hz时,虚部’’减小至趋于零的定值,表明KIO₃中的弛豫极化不再跟随外加电场变化。当样品处于P1相和HP-Ⅱ相时,tanδ随着频率的增加而降低;样品处于HP-Ⅰ相时,tanδ在一定频率出现弛豫峰。此时,电荷载流子的跳跃频率等于外加电场的频率。通过对比加压过程中7.8 GPa和卸压过程中7.6 GPa的介电损耗tanδ,发现卸压后KIO₃的介电损耗明显低于加压过程的介电损耗,KIO₃的介电性质在加压与卸压过程中得到了改善。更多还原

【Abstract】 KIO₃is a typical perovskite ferroelectric material,which can be used to manufacture nonlinear optical devices with good optical quality because of its large nonlinear optical coefficient and piezoelectric coefficient.As a thermodynamic parameter independent of temperature and chemical composition,pressure can tune the interatomic spacing and interatomic bonding patterns,thereby inducing crystal structure phase transitions and electronic structure phase transitions.After the phase transition,the material will exhibit novel properties that do not exist under normal pressure,thus expanding the research and application fields of materials.At present,the experimental research of KIO₃mostly focuses on the relationship between the properties and temperature,and the research on the properties of KIO₃under high pressure is not very comprehensive.Therefore,in this paper,using the diamond anvil cells device,combined with the in-situ AC impedance spectroscopy measurement technology and the first-principles calculation method,we have systematically studied the energy band structure,electronic density of states,conduction mechanism,and dielectric properties of KIO₃materials under high pressure.The purpose is to understand the electrical transmission behavior of KIO₃materials under high voltage.The research results are as follows:1.The atmospheric pressure X-ray diffraction experiments show that the atmospheric structure of the KIO₃material in this study is a triclinic structure,and the space group is P1.Through first-principles calculations,the band gap width of KIO₃under normal pressure is 3.066 e V.The top of the valence band is flat,and the bottom of the conduction band is relatively dispersed,so the delocalization of the conduction band is stronger.In the P1 phase,the band gap value of the KIO₃material decreases with increasing pressure.It is easier for electrons to transition from the valence band to the conduction band,so the resistance of the electron is smaller.By analyzing the density of electron states,it is found that the top and bottom valence bands are from O（2p）and I（2p）states,respectively.There is a strong hybridization between the s orbital and the p orbital of the O and I atoms.By analyzing the electron local function,it is found that the covalent bond is formed between O atom and I atom.When the pressure is 4 GPa,the electrons around the O atom are squeezed and migrate to the interstitial positions of the O and I atoms.So the material has electron conduction.2.It can be seen from the AC impedance spectrum under high pressure:when the pressure is less than 5.6 GPa,the conduction mechanism of KIO₃material is mixed conduction of ions and electrons.Between 4.3 and 5.6 GPa,the low-frequency region of the AC impedance spectrum has a sloping tail line representing the ion diffusion process,indicating that the pressure changes the grain boundary microstructure and makes the ion diffusion easier.When the pressure is between 5.6 and 20.6 GPa,the KIO₃material exhibits an ion conduction mechanism.When the pressure is increased to 10.0 GPa,the dip of the rising line of the impedance spectrum is almost zero,and the diffusion of ions becomes difficult.3.Using the equivalent circuit to fit the AC impedance spectrum,it is found that the grain boundary and grain resistance have abrupt changes at 4.3 GPa and 10.0 GPa,corresponding to the piezo-induced structural phase transition of KIO₃.In the range of0～20.6 GPa,the grain resistance is always smaller than the grain boundary resistance,and the grain boundary contributes more to the total resistance of the material.By studying the relationship between the relative permittivity,the real part,imaginary part and dielectric loss of KIO₃dielectric constant and frequency,it is found that when the pressure is less than 4.3 GPa,the relative permittivity gradually increases under the action of pressure.Because the electron locality around the O atom is reduced.When the frequency of the alternating electric field is greater than 10 Hz,the imaginary part decreases to a constant value that tends to zero.It is shown that the relaxation polarization in KIO₃no longer follows the applied electric field.When the sample is in P1 phase and HP-II phase,it decreases with the increase of frequency.When the sample is in HP-I phase,a relaxation peak appears at a certain frequency.At this time,the hopping frequency of the charge carriers is equal to the frequency of the applied electric field.By comparing the dielectric loss of 7.8 GPa in the pressurization process and 7.6 GPa in the decompression process,it is found that the dielectric loss of KIO₃after decompression is significantly lower than that of the pressurization process.The dielectric properties of KIO₃are improved during compression and decompression.更多还原