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Abstract:
Factors affecting the structure and orientation of the crystallographic shear (CS) planes in anion-deficient perovskites were investigated using the (Pb1-,Srz),-xFe(1+x),,, "perovskites as a model system. The isovalent substitution of Sr2+ for Pb2+ highlights the-influence of the A cation electronic strUcture because these cations exhibit very close ionic radii. Two compositional ranges have been identified in the system: 0.05 <= z <= 0.2, where the CS plane orientation gradually varies but stays close to (203)p, and 0.3 <= z <= 0.45 with (101)p CS,planes. The incommensurately modulated structure of Pb0.792Sro.,68Fel.0,002.629 was refined from neutron powder diffraction data using the (3+ 1)D approach (space group X2/rn(alpha 0(gamma)), X -= C / 2, V2, I/2), a = 3.9512(1) A, b = 3.9483(1) A, c =3.9165(1) k, = 93.268(2)6, q =0.0879(1)a* +0.1276(1)c*, R-F = 0.023, R-p = 0.029, and T = 900 K). A comparison of the compounds with different CS planes indicates that the orientation of the CS planes is governed mainly by the stereochemical "activity of the lone-electron-pair cations inside the perovskite blocks.
