Monoclinic and triclinic phases in higher-order Devonshire theory
Monoclinic and triclinic phases in higher-order Devonshire theory
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DOI：
10.1103/physrevb.63.094108
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发表时间：
2001-03-01
影响因子：
3.7
通讯作者：
Cohen, MH
中科院分区：
文献类型：
--
作者：
Vanderbilt, D;Cohen, MH
Devonshire理论通过对极序参数中自由能的六级展开，成功地描述了许多立方钙钛矿铁电材料，如BaTiO_3。然而，Noheda等人最近在PZT系统中发现了一种新的单斜铁电相。[APPL.太棒了。让我们来吧。74,2059(1999)]对这一理论提出了挑战。在这里，我们确认了六阶Devonshire理论不能支持单斜相，并考虑将该理论推广到更高阶。我们证明了八阶理论允许三种平衡相，其中极化不被限制在对称轴上，而是限制在对称平面上。其中一个阶段提供了新观察到的单斜晶相的自然描述。此外，该理论对单斜相、四方相和三方相之间的相边界的性质进行了可检验的预测。最低(三斜)对称型的铁电相，其中极化不受对称性的限制，直到德文希尔理论被推广到十二级才出现。对自由能表面临界点的拓扑分析有助于相变序列的讨论。
Devonshire theory provides a successful phenomenological description of many cubic perovskite ferroelectrics such as BaTiO3 via a sixth-order expansion of the free energy in the polar order parameter. However, the recent discovery of a novel monoclinic ferroelectric phase in the PZT system by Noheda et al. [Appl. Phys. Lett. 74, 2059 (1999)] poses a challenge to this theory. Here, we confirm that the sixth-order Devonshire theory cannot support a monoclinic phase, and consider extensions of the theory to higher orders. We show that an eighth-order theory allows for three kinds of equilibrium phases in which the polarization is confined not to a symmetry axis but to a symmetry plane. One of these phases provides a natural description of the newly observed monoclinic phase. Moreover, the theory mal;es testable predictions about the nature of the phase boundaries between monoclinic, tetragonal, and rhombohedral phases. A ferroelectric phase of the lowest (triclinic) symmetry type, in which the polarization is not constrained by symmetry, does not emerge until the Devonshire theory is carried to twelfth order. A topological analysis of the critical points of the free-energy surface facilitates the discussion of the phase transition sequences.