Probing Octahedral Tilting in Dion-Jacobson Layered Perovskites With Neutron Powder Diffraction and Raman Spectroscopy
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Probing Octahedral Tilting in Dion-Jacobson Layered Perovskites With Neutron Powder Diffraction and Raman Spectroscopy Joshua A. Kurzman and Margret J. Geselbracht Chemistry, Reed College, 3203 SE Woodstock Blvd., Portland, OR, 97202
ABSTRACT Two new Dion-Jacobson type layered perovskite solid solutions, RbCa2-xSrxM3O10 (M = Nb, Ta; 0 ≤ x ≤ 2), were prepared and studied by X-ray powder diffraction, neutron powder diffraction, and Raman spectroscopy. X-ray powder diffraction confirmed single-phase solid solution formation with continuous expansion of the idealized primitive tetragonal unit cell with increasing strontium content. Neutron powder diffraction studies of selected samples revealed lower symmetries and larger unit cells, as necessitated by octahedral tilting within the perovskite slabs, compared to the idealized primitive cell. As the average size of the A-cation in the perovskite slab is varied from Sr2+ to Ca2+, more extensive octahedral tilting is introduced. Vibrational modes of the perovskite slab observed using Raman spectroscopy show subtle changes as a function of calcium/strontium content and more intriguing differences between the isostructural niobates and tantalates. INTRODUCTION Layered perovskites provide a rich diversity of structures, compositions, chemical reactivity, and physical properties. Yet unlike the wealth of structural information that exists for three-dimensional perovskites, there is a paucity of detailed structural analyses for layered perovskites, particularly those containing early transition metals. In 3-D perovskites such as ABO3, the well-known tolerance factor, t, provides one means of predicting structural distortions induced by a mismatch in the ionic radii of the A and B cations within the ideal cubic oxide lattice. Thus, while SrTiO3 exhibits an ideal cubic perovskite structure, the smaller calcium ion in CaTiO3 induces distortions and tilting in the corner-shared TiO6 octahedral framework that lowers the symmetry to orthorhombic at room temperature [1]. While one would expect similar distortions in the perovskite slab of layered perovskites such as RbCa2Nb3O10, definitive evidence of such a distorted structure is lacking for this material. First prepared in 1981 [2], RbCa2Nb3O10 is assumed to be isostructural to CsCa2Nb3O10 although preliminary diffraction studies on single crystals of RbCa2Nb3O10 suggested a different space group [3]. The structure of CsCa2Nb3O10 determined by single crystal X-ray diffraction studies is shown on the left in Figure 1 and consists of triple-layer perovskite-type slabs interleaved by layers of cesium ions [4]. This is one of only a few studies demonstrating octahedral tilting within the perovskite slabs of an early transition metal layered perovskite; the complex pattern of tilting presumably optimizes the calcium-oxygen bonding arrangement. In addition to the tilting, the Nb5+ cation is too small for the octahedral hole in this structure and moves off-center, resulting in highly distorted NbO6 octahedra displaying a rang
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