Crystal structure determination of Ca5Nb5O17

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 have attracted attention because of their interesting physical and electronic properties. Some of these materials are high Tc ferroelectrics, while others are quasi-1D metals which show metal-semiconductor transition at low temperatures [1,2,3]. The latter is discussed in terms of a Peierls transition and charge-density wave, but their occurrence has not been definitely established yet. The basic structure of these compounds ca be derived from an ideal perovskite by stacking layers of corner-sharing BO6 octahedra. Consecutive slabs of octahedral, the thickness of which is determined by n, are displaced with respect to each other by about one-half the body diagonal of the octahedron and are separated by an additional layer of oxygen. In this study, crystals of pentacalcium pentaniobium heptadecaoxide (Ca5Nbn5O17) ( n = 5) were grown by floating-zone melting and the crystal structure at room temperature was determined by single-crystal x-ray diffraction with synchroton radiation using a CCD area detector. The compound crystallized in P21/c with a = 7.735(1) Å, b = 5.4851(5) Å, c = 32.171(17) Å β = 96.83(3) o. structure refinement indicates that the crystal is twinned. The coordination of Nb atoms reveals that the NbO6 octahedra in the middle of the slabs are the least distorted while those at the borders are the ones which are most deformed. Computed valences for Nb5+ also suggest that the extra electron in the 4d orbital of Nb are most likely found in the middle of the octahedral slabs