Layered perovskite CaNb0.8Ti0.2O3.33

Compounds of the homologous series AnBnO3n+2 (where A = Ca, La or SR and B = Ti or NB) with n = 4, 4.5, 5, 6, and 7 are interesting because of their physical and electronic properties. For some of them a charge density wave (CDW) has been proposed to develop at low temperatures, while others are ferroelectric materials. The ideal structure of these layered compounds, which can be derived from the perovskite structure, can be visualized as two alternating blocks of vertex-sharing BO6 octahedra separated by additional oxygen layer with A cations residing in the interblock cavities. The octahedral blocks are stacked along [110] of the ideal perovskite and their thickness is characterized by n. Consecutive blocks of octahedral are shifted with respect to each other by about half the octahedron body diagonal and B-O bond length leaving intrablock regions where the octahedral do not share two O atoms. In this study, crystals of CaNb0.8Ti0.2O3.33 (n=6) were grown by floating-zone melting and the crystal structure at room temperature was determined by single-crystal x-ray diffraction with synchrotron radiation using a CCD area detetctor. The compound crystallized with a monoclinic unit cell with a = 7.681(2) Å, b = 5.465(1) Å, c = 37.747(6) Å β = 95.87(1) o. The crystal structure was determined by direct methods using SIR2002, and the structure was refined using JAN2000. Particular attention is given to the different distortions of the (Nb,Ti)O6 octahedra near the center and on the borders of the slabs, the Nb,Ti order, and the different environments of the Ca atoms