Batuk Dmitry, Batuk Maria, Filimonov Dmitry S, Zakharov Konstantin V, Volkova Olga S, Vasiliev Alexander N, Tyablikov Oleg A, Hadermann Joke, Abakumov Artem M
Electron Microscopy for Material Science (EMAT), University of Antwerp , Groenenborgerlaan 171, B-2020 Antwerp, Belgium.
Chemistry Department, Moscow State University , 119991 Moscow, Russia.
Inorg Chem. 2017 Jan 17;56(2):931-942. doi: 10.1021/acs.inorgchem.6b02559. Epub 2016 Dec 23.
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.
BiTiFeO材料由厚度为n个(Ti,Fe)O八面体的(001)面平行钙钛矿块构成，这些钙钛矿块由周期性平移界面分隔。界面基于边共享(Ti,Fe)O八面体的类锐钛矿链。它们与钙钛矿块的八面体一起，形成了由孤对Bi阳离子稳定的S形隧道。在这项工作中，使用先进的透射电子显微镜、粉末X射线衍射和穆斯堡尔光谱对n = 4 - 6的BiTiFeO同系物的结构进行了详细分析。类锐钛矿链与钙钛矿块的连接导致沿界面具有3a的周期性，使得它们可以彼此堆叠或沿[100]方向有±a的位移。界面的有序排列产生了正交Immm和单斜A2/m多晶型，其晶胞参数分别为a = 3a、b = b、c = 2(n + 1)c和a = 3a、b = b、c = 2(n + 1)c - a。虽然n = 3的化合物是正交的，但单斜变体在更高的同系物中更有利。BiTiFeO结构在钙钛矿块中表现出复杂的原子位移模式，这由Bi阳离子的立体化学活性所支持。这些模式与钙钛矿块边界处(Ti,Fe)O层中氧原子的阳离子配位相关。这种耦合在n = 3、4的同系物中很强，但随着钙钛矿块厚度的增加而逐渐减弱，因此在n = 6的化合物中，原子位移的主导模式沿界面平面排列。相邻钙钛矿块中的位移倾向于反平行排列，从而形成整体的反极结构。BiTiFeO材料表现出异常多样的结构缺陷。n = 4 - 6的同系物分别在T = 135、220和295 K以下是强反铁磁体。它们表现出高介电常数，该常数随温度微弱增加，并且对Ti/Fe比相对不敏感。
