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Features of the SrLa2Sc2–xInxO7 two-slab structure
Authors: Y.A. Titov, N.M. Belyavina, M.S. Slobodyanik, V.V. Chumak, O.I. Nakonechna
Number of views: 234
The conditions of isovalent substitution of scandium atoms for indium atoms in B position of the SrLa2Sc2O7 two-slab perovskite structure of the SrLa2Sc2-xInxO7 type (0x0.4) are determined by X-ray powder diffraction methods. A destruction of the two-slab structure and formation of the second phase with the rhombic LaInO3 structure, which is dominant at x=0.8–2.0, are observed at a further increase of the indium atoms content in the SrLa2Sc2–xInxO7 phases. Orthorhombic crystal structure (Fmmm space group) of the SrLa2Sc2–xInxO7 phases with the scandium atoms substitution degrees of 0.2 and 0.4 is determined by the Rietveld procedure. The SrLa2Sc2-xInxO7 crystal structure is formed by two-dimensional (infinite in the XY plane) perovskite-like blocks, which consists of two slabs of (Sc,In)O6 deformed octahedra. The (Sc,In)O6 octahedra are connected only by vertices, and each (Sc,In)O6 octahedron has five common vertices with the adjacent octahedra of the same block. There is no direct connection between the (Sc,In)O6 octahedra of adjacent perovskite-like blocks in the SrLa2Sc2–xInxO7 slab structure. Adjacent blocks are separated by a slab of (Sr,La)2O9 polyhedra and are held together by means of –O–(Sr, La)2–O– bonds. The eight oxygen atoms of the (Sr,La)2O9 polyhedron belong to the same block as the (Sr,La)2 atoms and the one oxygen atom belongs to the neighboring block. Coordination number of the (Sr,La)1 intrablock atoms is equal to 12, and their coordination polyhedron is a deformed cuboctahedron. The distribution of strontium and lanthanum atoms over the SrLa2Sc2–xInxO7 structure positions has a partially ordered character with the predominant localization of strontium atoms in the intrablock cuboctahedral voids of the perovskite-like block. Analysis of the structural parameters of the SrLa2Sc2–xInxO7 phases shows that a gradual decrease in the length of the(Sr,La)2–O2 interblock bonds (from 0.222(2) nm (x=0) to 0.213(1) nm (x=0.4)) and an increase in the degree of deformation of the (Sr,La)2O9 interblock polyhedra (from 6810–4 (х=0) to 9510–4 (х=0.4)) take place with the isovalent substitution of scandium atoms for large indium atoms in the slab structure of the SrLa2Sc2–xInxO7. Reducing the perovskite-like blocks interval makes a two-dimensional slab structure closer to the structure of a three-dimensional perovskite, and an increase in the deformation degree of the (Sr,La)2О9 interblock polyhedra leads to an increase in the tension of the interblock space. These changes result in destabilization and destruction of the slab perovskite-like structure of the SrLa2Sc2–xInxO7 phases, limit the solid solutions range and cause the absence of SrLa2In2O7 indate.