【作者】 吴忠庆；

【导师】 顾秉林； 段文晖；

【作者基本信息】 清华大学 ， 凝聚态物理， 2003， 博士

【摘要】 在本论文中，我们从理论上研究了弛豫铁电体的介电特性。 我们用Monte Carlo方法研究了缺陷导致的无规内场对介电常数的影响，结果显示仅在峰值温度附近时介电常数才明显受到内场的影响。介电常数随内场变化经历了小内场时的几乎不变，然后是随内场线性减小。这样一种变化关系跟介电常数随老化时间的对数关系非常类似。这在一定程<WP=4>度上预示弛豫铁电体的内场是随时间对数线性增加的。在不同温度下，介电常数改变量跟测量频率的曲线特征是不一样的，在低温下曲线是凹的，升高温度，曲线过渡到凸的。这些结果与老化率的实验一致。我们进一步用Monte Carlo方法模拟了弛豫铁电体薄膜的介电特性。如果不包含表面效应，介电常数并不随薄膜厚度变化。因此可以肯定是表面效应导致薄膜的介电常数比体材料低两个数量级。通过增加一个表面层，我们发现随着薄膜变薄，峰值介电常数减小而相应的峰值温度增加。峰值温度以上也能观察到明显的色散现象。这些特点都与薄膜实验相符。通过引进各向同性分布的无规场和近邻隧穿，进一步发展了八势阱有序无序模型，并用它研究了弛豫铁电体的场诱导相变和极化行为。我们发现，铁电相变的类型（一级或二级）不随无规场而变。反向势阱之间的隧穿促进场诱导相变，而近邻势阱之间的隧穿则抑制场诱导相变。当加上一个适当的近邻势阱之间的隧穿频率，外电场下的极化温度行为，包括一个临界电场和相应的非零的相变温度，能很好的与弛豫铁电体的场致相变相符。这表明在弛豫铁电体中近邻隧穿是比较重要的。弥散相变是弛豫铁电体的一个主要特征。我们用八势阱有序无序模型研究了表征弥散度的临界指数随隧穿频率的依赖关系。对于低隧穿频率情形，铁电相变是二级的，体系服从居里－外斯关系。进一步增加频率到一级相变区，临界指数开始偏离1，并随频率增加而增大。显示体系越来越弥散。我们的结果有助于理解一些复合钙钛矿铁电体在一定压强下转变为弛豫铁电体的现象。更多还原

【Abstract】 The dielectric properties of relaxor ferroelectrics are theoretically investigated in this dissertation.The effect of defect-induced internal field on the dielectric response of relaxor ferroelectrics is investigated by Monte Carlo simulation. Only at a small temperature range nearby the temperature of the dielectric maximum does the susceptibility decrease markedly due to the internal field. The susceptibility is almost independent of<WP=5>the internal field width at low internal field width, and then decreases linearly with enhancing internal field width. This dependence of the susceptibility on the internal field width is very similar to the relation of dielectric constant against logarithmic aging time, which probably suggests a linear dependence of the internal field on the logarithm of the aging time. The frequency dependence of the susceptibility aging is sensitive to the temperature. With increasing temperature, the curve of the susceptibility change against logarithmic frequency varies from concave to approximately linear, and then to convex, which is in agreement with the recent aging rate measurement.The dielectric properties of relaxor film are studied by Monte Carlo simulation. The dielectric constant does not vary with the film thickness if no surface effect is included. Therefore, the phenomena that the dielectric constant in the film is much smaller than that in the bulk by almost two orders of magnitude, should be attributed to the surface effect. By adding a surface layer in simulation, we has observed a decrease in the dielectric maximum and increase in temperature of the dielectric maximum as the thickness is decreased. The frequency dispersion of the dielectric constant at temperature above the temperature of the dielectric maximum has also been obtained. These results are consistent with experiments. By generalizing the eight-potential-well model by introducing isotropic random fields and considering two kinds of tunnelings, we investigated field-induced phase transition and polarization behaviour of ferroelectrics under an external field. The type of the phase transition, the first-order or second-order, is almost independent of the random field width. We found that the tunneling frequencies between wells with opposite directions prompts the field-induced phase transition while the tunneling frequencies between the nearest-neighbor wells inhibits the field-induced transition. When a moderate nearest-neighbor well tunneling frequency is considered, the behavior of average polarization with temperature at external field, including the existence of the threshold field and the corresponding temperature, can be well described for the relaxors with field-induced phase transition. Many important features of field-induced phase transition and the behavior of polarization with temperature are in agreement with the experimental measurements. The diffuse phase transition is a main characteristic of typical relaxor ferroelectrics. Dependence of the critical exponent which denote the diffuseness on the tunneling frequency has been investigated with the eight-potential-well order-disorder ferro- electric model. For low tunneling frequency, the ferroelectric phase transition is of second-order and the system conforms to the Curie-Weiss law. The critical exponent departs significantly from the Curie-Weiss value of unity and increases with increasing tunneling frequency at the region of the first-order phase transition Our work is useful<WP=6>for understanding the phenomena that some normal ferroelectrics show pressure induced crossover from normal ferroelectric to relaxor.更多还原