Effect of isovalent substitution of lanthanum atoms on the slab structure of ВаLa1- xNdxInO4 indates

Abstract

The boundary of the isovalent substitution of a lanthanum atom by a neodymium atom in the slab perovskitelike structure of ВаLa_{1 – x}Nd_xInO₄ indates (0 ⩽ х ⩽ 0.25) has been determined using the X-ray powder diffraction methods. The crystal structure calculations were performed by use of the software package containing a full complex of the Rietveld procedures, including the crystal structure determination in multiphase samples. As a result, it was shown that, in fact, the ВаLa_{1 – x}Nd_xInO₄ (x = 0.2) synthesis product contains two polymorphic modifications of a slab perovskite-like structure, namely: the main orthorhombic ВаLa_0,8Nd_0,2InO₄ phase with space group Pbca and the additional tetragonal one with space group I4/mmm. It was shown that the crystal structure of each ВаLa_{1 – x}Nd_xInO₄ modification is formed by the two-dimensional perovskite-like blocks constructed from a slab of distorted InO6 octahedra joined by their vertices. These blocks are separated by the slab of (Ba,La,Nd)О₉ polyhedra. Here, the blocks are connected through —O—(Ва,La,Nd)—O— bonds, while there are no direct In—O—In bonds between the octahedra of adjacent blocks. Analysis of the structural features of the orthorhombic ВаLa_0,8Nd_0,2InO₄ modification has shown that the isovalent substitution of lanthanum atoms by smaller-size neodymium atoms leads as to a significant (more than one and a half times) increase in the deformation degree of the interblock (Ba,La,Nd)O₉ polyhedra, as well to a decrease in the (Ba,La,Nd)—О₂ interblock length. Such structural changes result in a destabilization of the interblock “stitching” and become one of the main destruction factors of the slab perovskite-like structure of the ВаLa_{1 – x}Nd_xInO₄ phases at x > 0.25 and cause the absence of the BaNdInO₄ indate