Local Structure of Disordered Bi0.5K0.5TiO3 Investigated by Pair Distribution Function Analysis and First-Principles Calculations
Local Structure of Disordered Bi0.5K0.5TiO3 Investigated by Pair Distribution Function Analysis and First-Principles Calculations
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DOI：
10.1021/acs.chemmater.7b00276
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发表时间：
2017-05-23
影响因子：
8.6
通讯作者：
Selbach, Sverre M.
中科院分区：
文献类型：
--
作者：
Jiang, Bo;Grande, Tor;Selbach, Sverre M.
利用密度泛函理论（DFT）计算和同步加速器x射线全散射研究了铁电钙钛矿Bi0.5K0.5TiO3 （BKT）中a位阳离子的有序性。利用BKT作为具有混合a位阳离子的原型无铅铁电钙钛矿，我们使用理论和实验相结合的方法来评估阳离子排序的能量学和由此产生的物理性质。利用实际空间对分布函数（pdf）和DFT计算，对2 × 2 × 2超级单体中10种不同的Bi/K构型进行了评价。从实验或理论来看，这些结构都没有特别有利。采用Berry相法计算的铁电极化值为50-105 μ C/cm(2)。这明显大于之前报道的22-49 μ C/cm(2)的实验结果，表明BKT不具有远程A位阳离子顺序。对一个12 × 12 × 12的超级单体的总散射数据进行反向蒙特卡罗（RMC）模拟也支持了BKT中大量的a位紊乱。在伪无序A位亚晶格中，具有局部阳离子位移的4 × 4 × 4超级单体重现了实验观察到的极化现象，这意味着在实际材料中存在多个局部极性区域，这些区域部分相互抵消。将pdf的RMC建模与DFT计算相结合，可以高度适用于其他具有亚晶格无序的晶体材料。
We investigate A-site cation ordering in the ferroelectric perovskite Bi0.5K0.5TiO3 (BKT) by density functional theory (DFT) calculations and synchrotron X-ray total scattering. Using BKT as a prototypical lead-free ferroelectric perovskite with mixed A-site cations, we use a combination of theory and experiments to assess the energetics and resulting physical properties of cation ordering. Ten different Bi/K configurations in a 2 x 2 x 2 supercell were assessed by real space pair distribution functions (PDFs) and DFT calculations. None of these configurations were identified as particularly favorable from experiment or theory. Ferroelectric polarization calculated by the Berry phase method for all ten configurations yields values of 50-105 mu C/cm(2). This is significantly larger than previously reported experimental results in the range of 22-49 mu C/cm(2), indicating that BKT does not possess long-range A site cation order. Reverse Monte Carlo (RMC) modeling of the total scattering data with a 12 x 12 x 12 supercell also supports substantial A-site disorder in BKT. A 4 x 4 x 4 supercell with local cation displacements in a pseudodisordered A-site sublattice reproduces the experimentally observed polarization, implying that in a real material there are multiple local polar regions which partly cancel each other. The combination of RMC modeling of PDFs with DFT calculations should be highly applicable to other crystalline materials with sublattice disorder.