外延BaxSr（1-x）TiO3薄膜相变的唯象理论
Phenomenological theory of phase transitions in epitaxial BaxSr(1-x)TiO3 thin films
摘要：
采用Landau-Devonshire方法建立了立方晶系衬底上外延生长钛酸锶（1-x）TiO 3（BST-x）薄膜的唯象热力学理论。为了开发BST- $x$固溶体在高温下有效的热力学势，采用BT单晶的八阶热力学势和ST单晶的修正的四阶热力学势作为固溶体端元的起始势。为了使理论相图与实验相图相吻合，改变了BT的几个势能系数。对于低Ba含量，我们构造了特定的相图，其中五个相会聚在多相点（T_N ~ 2 = 47 K，x = 0.028），所有的相变都是二级的。绘制并讨论了BST-x薄膜室温下的“浓度-失配应变”相图和特定浓度下的“温度-失配应变”相图。在T_N2附近，外延膜中极化与结构序参量之间的耦合被显著地改变，并且在相图上出现了大量体材料中不存在的新相。
英文摘要：
A phenomenological thermodynamic theory of BaxSr(1-x)TiO3 (BST-x) thin films epitaxially grown on cubic substrates is developed using the Landau-Devonshire approach. The eighth-order thermodynamic potential for BT single crystal and modified fourth-order potential for ST single crystal were used as starting potentials for the end-members of the solid solution with the aim to develop potential of BST-$x$ solid solution valid at high temperatures. Several coefficients of these potentials for BT were changed to obtain reasonable agreement between theory and experimental phase diagram for BST-x (x > 0.2) solid solutions. For low Ba content we constructed the specific phase diagram where five phases converge at the multiphase point (T_N2 = 47 K, x = 0.028) and all transitions are of the second order. The "concentration-misfit strain" phase diagrams for BST-x thin films at room temperature and "temperature-misfit strain" phase diagrams for particular concentrations are constructed and discussed. Near T_N2 coupling between polarization and structural order parameter in the epitaxial film is modified considerably and large number of new phases not present in the bulk materials appear on the phase diagram.
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