Antiferroelectric (Pb,Bi)1-xFe1+xO3-y Perovskites Modulated by Crystallographic Shear Planes

We demonstrate for the first time a possibility to vary the anion content in perovskites over a wide range through a long-range-ordered arrangement of crystallographic shear (CS) planes. Anion-deficient perovskites (Pb,Bi)(1-x)Fe1+xO3-y with incommensurately modulated structures were prepared as single phases in the compositional range from Pb0.857Bi0.094Fe1.049O2.572 to Pb0.409Bi0.567Fe1.025O2.796. Using a combination of electron diffraction and high-resolution scanning transmission electron microscopy, we constructed a superspace model describing a periodic arrangement of the CS planes. The model was verified by refinement of the Pb0.64Bi0.32Fe1.04O2.675 crystal structure from neutron powder diffraction data ((3 + 1)D S.G. X2/m(alpha 0 gamma), X = [1/2,1/2,1/2,1/2], a = 3.9082(1) angstrom, b = 3.90333(8) angstrom, c = 4.0900(1) angstrom, beta = 91.936(2)degrees, q = 0.05013(4)a* + 0.09170(3)c* at T = 700 K, R-P = 0.036, R-wP = 0.048). The (Pb,Bi)(1-x)Fe1+xO3-y structures consist of perovskite blocks separated by CS planes confined to nearly the (509)(p) perovskite plane. Along the CS planes, the perovskite blocks are shifted with respect to each other over the 1/2[110](p) vector that transforms the corner-sharing connectivity of the FeO6 octahedra in the perovskite framework to an edge-sharing connectivity of the FeO5 pyramids at the CS plane, thus reducing the oxygen content. Variation of the chemical composition in the (Pb,Bi)(1-x)Fe1+xO3-y series occurs mainly because of a changing thickness of the perovskite block between the interfaces, that can be expressed through the components of the q vector as Pb6 gamma+2 alpha Bi1-7 gamma-alpha Fe1+gamma-alpha O3-3 gamma-alpha. The Pb, Bi, and Fe atoms are subjected to strong displacements occurring in antiparallel directions on both sides of the perovskite blocks, resulting in an antiferroelectric-type structure. This is corroborated by the temperature-, frequency-, and field-dependent complex permittivity measurements. Pb0.64Bi0.32Fe1.04O2.675 demonstrates a remarkably high resistivity > 0.1 T Omega cm at room temperature and orders antiferromagnetically below T-N = 608(10) K.