Specific features of nonvalent interactions in orthorhombic perovskites
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TAL CHEMISTRY
Specific Features of Nonvalent Interactions in Orthorhombic Perovskites V. N. Serezhkin, D. V. Pushkin, and L. B. Serezhkina Samara State University, Samara, 443011 Russia email: [email protected] Received July 22, 2013
Abstract—It is established that isostructural orthorhombic perovskites ABO3 (sp. gr. Pnma in different sys tems, no. 62, Z = 4), depending on the specificity of nonvalent interactions (which determine the combina torial–topological type of the Voronoi–Dirichlet polyhedra (VDPs) of four basis atoms), are divided into ten different stereotypes. It is shown by the example of 259 perovskites belonging to the DyCrO3 stereotype that VDP characteristics can be used to quantitatively estimate the distortion of BO6 octahedra, including that caused by the Jahn–Teller effect. It is found that one of the causes of the distortion of the coordination poly hedra of atoms in the structure of orthorhombic perovskites is heteroatomic metal–metal interactions, for which the interatomic distances are much shorter than the sum of the Slater radii of A and B atoms. DOI: 10.1134/S106377451403016X
INTRODUCTION It is known that compounds of the ABX3 composi tion (A and B are metals and Х is a nonmetal) belong most often to the structural perovskite family. In the idealperovskite structure, B atoms occupy vertices of a primitive cubic unit cell, whereas Х atoms are located at the centers of all cube edges. As a result, B atoms form BX6 octahedra, which are connected (via bridge Х atoms) into a threedimensional framework of the BX3 = BX6/2 composition. Each A atom is located at the center of the cubic cell and coordinated by 12 Х atoms. In the ideal perovskite structure, A, B, and Х atoms have, respectively, Oh, Oh, and D4h site symme tries and coordination numbers of 12, 6, and 6 (2B + 4A). Although cubic perovskites exist in nature (an example is SrFeO3), the structure of most ABX3 compounds (X = O, S, F, Cl, Br, H) was found to be a distorted ver sion of the aforementioned cubic structure [1–6]. In particular, the parent perovskite compound, СaTiO3, has an orthorhombic rather than cubic (as was sug gested previously) unit cell. To date, compounds with a perovskitetype structure, the real symmetry of which is reduced from cubic to triclinic, have been revealed. According to [1], the symmetry of an ideal cubic structure of ABX3 perovskite can be reduced in three ways: distortion of BX6 octahedra; displacement of the B atom from the octahedron center; or small rotation of octahedra around one, two, or all three C4 axes, which coincide with the translations а, b, and с of the initial cubic cell. The first two ways of distortion are due to the electronic instability of B atoms, caused by the Jahn–Teller effect (in particular, when B = Mn(III) or Сu(II) [2]) or their ability to form localized
multiple bonds (for example, Ti=O [7]). The third (and the most general) way of distortion, at which the bond topology in the threedimensional framework BX6/2 also remains invariable, is generally observed when
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