ABSTRACT A higher-order Landau–Ginzburg–Devonshire theory is proposed to describe phenomenologically the complex phase transitions in the MPB region of the relaxor ferroelectric (1−x)Pb(Mg1/3Nb2/3)O3-xPbTiO3(PMN-PT) single crystals. Using the singularity theory, a structurally stable 10th-order potential with four varying expansion coefficients (a1, a11, a2 and a12) depending on temperature and composition is found out. By studying on two- and three-dimensional sections of a four-dimensional phase diagram a1-a11-a2-a12 in the space of phenomenological parameters, the range of coefficient parameters by which the structural stability of the potential model can be guaranteed is determined. The theoretical temperature-composition phase diagram is constructed and it is shown that this is in good agreement with both recent experimental data and first-principle calculation for the symmetry change. Moreover, temperature dependence of dielectric constants for various compositions is studied.

中文翻译：

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(1-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3单晶MPB区相变的现象学理论

摘要 提出了一种高阶 Landau-Ginzburg-Devonshire 理论来从现象学上描述弛豫铁电体 (1-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3(PMN-PT) 单晶体。使用奇点理论，发现了具有四个不同膨胀系数（a1、a11、a2 和 a12）的结构稳定的 10 阶势，这取决于温度和成分。通过在唯象参数空间中研究四维相图a1-a11-a2-a12的二维和三维截面，确定了能保证势模型结构稳定性的系数参数范围. 构建了理论温度组成相图，结果表明这与最近的实验数据和对称性变化的第一性原理计算非常吻合。此外，研究了各种组合物的介电常数的温度依赖性。