Antiferroelectric (Pb,Bi)_1−xFe_1+xO_3−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 Fe 1+ x O 3− y with incommensurately modulated structures were prepared as single phases in the compositional range from Pb 0.857 Bi 0.094 Fe 1.049 O 2.572 to Pb 0.409 Bi 0.567 Fe 1.025 O 2.796 . 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 Pb 0.64 Bi 0.32 Fe 1.04 O 2.675 crystal structure from neutron powder diffraction data ((3 + 1)D S.G. X 2 /m (α0γ), X = [1/2,1/2,1/2,1/2], a = 3.9082(1) Å, b = 3.90333(8) Å, c = 4.0900(1) Å, β = 91.936(2)°, 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 Fe 1+ x O 3− 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 FeO 6 octahedra in the perovskite framework to an edge-sharing connectivity of the FeO 5 pyramids at the CS plane, thus reducing the oxygen content. Variation of the chemical composition in the (Pb,Bi) 1− x Fe 1+ x O 3− 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 Pb 6γ+2α Bi 1−7γ−α Fe 1+γ−α O 3−3γ−α . 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. Pb 0.64 Bi 0.32 Fe 1.04 O 2.675 demonstrates a remarkably high resistivity >0.1 T Ω cm at room temperature and orders antiferromagnetically below T N = 608(10) K.