Crystal Structure, Defects, Magnetic and Dielectric Properties of the Layered BiTiFeO Perovskite-Anatase Intergrowths.

The BiTiFeO materials are built of (001) plane-parallel perovskite blocks with a thickness of n (Ti,Fe)O octahedra, separated by periodic translational interfaces. The interfaces are based on anatase-like chains of edge-sharing (Ti,Fe)O octahedra. Together with the octahedra of the perovskite blocks, they create S-shaped tunnels stabilized by lone pair Bi cations. In this work, the structure of the n = 4-6 BiTiFeO homologues is analyzed in detail using advanced transmission electron microscopy, powder X-ray diffraction, and Mössbauer spectroscopy. The connectivity of the anatase-like chains to the perovskite blocks results in a 3a periodicity along the interfaces, so that they can be located either on top of each other or with shifts of ±a along [100]. The ordered arrangement of the interfaces gives rise to orthorhombic Immm and monoclinic A2/m polymorphs with the unit cell parameters a = 3a, b = b, c = 2(n + 1)c and a = 3a, b = b, c = 2(n + 1)c - a, respectively. While the n = 3 compound is orthorhombic, the monoclinic modification is more favorable in higher homologues. The BiTiFeO structures demonstrate intricate patterns of atomic displacements in the perovskite blocks, which are supported by the stereochemical activity of the Bi cations. These patterns are coupled to the cationic coordination of the oxygen atoms in the (Ti,Fe)O layers at the border of the perovskite blocks. The coupling is strong in the n = 3, 4 homologues, but gradually reduces with the increasing thickness of the perovskite blocks, so that, in the n = 6 compound, the dominant mode of atomic displacements is aligned along the interface planes. The displacements in the adjacent perovskite blocks tend to order antiparallel, resulting in an overall antipolar structure. The BiTiFeO materials demonstrate an unusual diversity of structure defects. The n = 4-6 homologues are robust antiferromagnets below T = 135, 220, and 295 K, respectively. They show a high dielectric constant that weakly increases with temperature and is relatively insensitive to the Ti/Fe ratio.