Empirical Evidence for A-Site Order in Perovskites

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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 (Na0.5La0.5TiO3) 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 Na0.5La0.5TiO3 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(Mg0.5W0.5)O3 solid solutions. These systems provide additional validation of the accuracy and versatility of the empirical modeling method used.


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