Empirical Evidence for A-Site Order in Perovskites

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• 作者：Kevin Tolman ; Rick Ubic ; Bing Liu ; Izaak Williamson ; Katherine Bedke ; Eric B. Nelson ; Lan Li and Xiang Ming Chen
• 刊名：Journal of the American Ceramic Society
• 出版年：2017
• 出版时间：January 2017
• 年：2017
• 卷：100
• 期：1
• 页码：429-442
• 全文大小：2312K
• ISSN：1551-2916

文摘

Models for composition–structure relationships are useful in both the lab and industry, yet few exist for perovskites-containing extrinsic defects or cation ordering. In this work, an empirical model is used to predict the existence of A-site cation ordering. Specifically, four compositions in the Na_(1−3x)/2La_(1+x)/2TiO₃ system (x = 0.0, 0.0533, 0.1733 and 0.225) were synthesized using a conventional solid-state mixed-oxide method. The structure of the x = 0 end-member (Na_0.5La_0.5TiO₃) has been reported in various space groups, but always with a random distribution of Na⁺ and La³⁺ on the A site; however, empirical modeling suggests that it is not only ordered but also that a small volume increase accompanies the ordering process. While no evidence of long-range A-site ordering is observed in this composition via X-ray or neutron diffraction, electron-diffraction data indicate short-range ordering of Na⁺ and La³⁺ ions, with the degree of cation ordering decreasing (but the scale of ordered domains and degree of vacancy ordering generally increasing) with increasing x. First-principles calculations via density functional theory support both conclusions that short-range ordering in Na_0.5La_0.5TiO₃ is stable and that it results in a volume increase with respect to the disordered analog. A similar analysis has been conducted for the Li_(1−3x)/2La_(1+x)/2TiO₃ and Na_(1−3x)/2La_(1+x)/2(Mg_0.5W_0.5)O₃ solid solutions. These systems provide additional validation of the accuracy and versatility of the empirical modeling method used.