Zhang Li-Li, Huang Yi-Neng
National Laboratory of Solid State Microstructures, School of Physics, Nanjing University, Nanjing, China.
Xinjiang Laboratory of Phase Transitions and Microstructures in Condensed Matters, College of Physical Science and Technology, Yili Normal University, Yili, China.
Sci Rep. 2020 Mar 19;10(1):5060. doi: 10.1038/s41598-020-61911-5.
Relaxor-ferroelectrics are fascinating and useful materials, but the mechanism of relaxor-ferroelectricity has been puzzling the scientific community for more than 65 years. Here, a theory of relaxor-ferroelectricity is presented based on 3-dimensional-extended-random-site-Ising-model along with Glauber-dynamics of pseudospins. We propose a new mean-field of pseudospin-strings to solve this kinetic model. The theoretical results show that, with decreasing pseudospin concentration, there are evolutions from normal-ferroelectrics to relaxor-ferroelectrics to paraelectrics, especially indicating by the crossovers from, (a) the sharp to diffuse change at the phase-transition temperature to disappearance in the whole temperature range of order-parameter, and (b) the power-law to Vogel-Fulcher-law to Arrhenius-relation of the average relaxation time. Particularly, the calculated local-order-parameter of the relaxor-ferroelectrics gives the polar-nano-regions appearing far above the diffuse-phase-transition and shows the quasi-fractal characteristic near and below the transition temperature. We also provide a new mechanism of Burns-transformation which stems from not only the polar-nano-regions but also the correlation-function between pseudospins, and put forward a definition of the canonical relaxor-ferroelectrics. The theory accounts for the main facts of relaxor-ferroelectricity, and in addition gives a good quantitative agreement with the experimental results of the order-parameter, specific-heat, high-frequency permittivity, and Burns-transformation of lead magnesium niobate, the canonical relaxor-ferroelectric.
弛豫铁电体是一类引人入胜且实用的材料，但其弛豫铁电性的机制在超过65年的时间里一直困扰着科学界。在此，基于三维扩展随机位点伊辛模型以及赝自旋的格劳伯动力学，提出了一种弛豫铁电性理论。我们提出了一种新的赝自旋弦平均场来求解这个动力学模型。理论结果表明，随着赝自旋浓度的降低，存在从普通铁电体到弛豫铁电体再到顺电体的演变，尤其表现为：(a) 序参量在相变温度处从急剧变化到弥散变化直至在整个温度范围内消失；(b) 平均弛豫时间从幂律关系到沃格尔 - 富尔彻定律再到阿仑尼乌斯关系。特别地，计算得到的弛豫铁电体的局域序参量表明，极性纳米区域出现在弥散相变温度之上很远的地方，并且在转变温度附近及以下呈现准分形特征。我们还提供了一种伯恩斯转变的新机制，它不仅源于极性纳米区域，还源于赝自旋之间的关联函数，并提出了典型弛豫铁电体的定义。该理论解释了弛豫铁电性的主要事实，此外，在序参量、比热、高频介电常数以及典型弛豫铁电体铌镁酸铅的伯恩斯转变等实验结果方面给出了良好的定量一致性。
