Crystal structure, defects, magnetic and dielectric properties of the layered Bi3n+1Ti7Fe3n−3O9n+11 perovskite-anatase intergrowthsстатья
Статья опубликована в высокорейтинговом журнале
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Дата последнего поиска статьи во внешних источниках: 24 марта 2017 г.
Аннотация:The Bi3n+1Ti7Fe3n−3O9n+11 materials are built of (001)p planeparallel
perovskite blocks with a thickness of n (Ti,Fe)O6 octahedra, separated by
periodic translational interfaces. The interfaces are based on anatase-like chains of
edge-sharing (Ti,Fe)O6 octahedra. Together with the octahedra of the perovskite
blocks, they create S-shaped tunnels stabilized by lone pair Bi3+ cations. In this
work, the structure of the n = 4−6 Bi3n+1Ti7Fe3n−3O9n+11 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 3ap periodicity along the interfaces, so
that they can be located either on top of each other or with shifts of ±ap along
[100]p. The ordered arrangement of the interfaces gives rise to orthorhombic
Immm and monoclinic A2/m polymorphs with the unit cell parameters a = 3ap, b
= bp, c = 2(n + 1)cp and a = 3ap, b = bp, c = 2(n + 1)cp − ap, respectively. While the n = 3 compound is orthorhombic, the monoclinic modification is more favorable in higher homologues. The Bi3n+1Ti7Fe3n−3O9n+11 structures demonstrate intricate patterns of atomic displacements in the perovskite blocks, which are supported by the stereochemical activity of the Bi3+ cations.
These patterns are coupled to the cationic coordination of the oxygen atoms in the (Ti,Fe)O2 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 Bi3n+1Ti7Fe3n−3O9n+11 materials demonstrate an unusual diversity of structure defects. The n = 4−6 homologues are robust antiferromagnets below TN = 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.