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Empirical Evidence for A‐Site Order in Perovskites
Author(s) -
Tolman Kevin,
Ubic Rick,
Liu Bing,
Williamson Izaak,
Bedke Katherine,
Nelson Eric B.,
Li Lan,
Chen Xiang Ming
Publication year - 2017
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/jace.14547
Subject(s) - neutron diffraction , vacancy defect , crystallography , ion , solid solution , order (exchange) , diffraction , materials science , density functional theory , chemistry , crystal structure , physics , computational chemistry , organic chemistry , finance , economics , optics , metallurgy
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 (1−3 x )/2 La (1+ x )/2 TiO 3 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 0.5 La 0.5 TiO 3 ) has been reported in various space groups, but always with a random distribution of Na + and La 3+ 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 La 3+ 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 0.5 La 0.5 TiO 3 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 (1−3 x )/2 La (1+ x )/2 TiO 3 and Na (1−3 x )/2 La (1+ x )/2 (Mg 0.5 W 0.5 )O 3 solid solutions. These systems provide additional validation of the accuracy and versatility of the empirical modeling method used.