Reinvestigation of the crystal structure of pentastrontium tetraniobate, Sr5Nb4O15

The crystal structure of Sr5Nb4O15, achieved by Rietveld analysis of X-ray powder diffraction pattern using the structural parameters of Ba5Ta4O15 (trigonal symmetry, space group P (3) over bar ml), results of a monoclinic distortion of the hexagonal cell [1]. However the refinement is quite poor: no R value, no thermal factors and no justification of choosing the monoclinic system were given. Moreover many atomic distances (mainly Nb-O and O-O) are anomalous with respect to the well-known crystal chemistry of niobates. The reinvestigation was done from single crystal X-ray diffraction data and preliminary electron diffraction analysis of the reciprocal lattice. It is definitively a member n = 5 of the homologous series of complex oxides with cation-deficient perovskite-related structure with trigonal symmetry formulated as A(n)B(n-1)O(3n). The P (3) over bar cl space group chosen instead that of Ba5Ta4O15 [2, 3] corresponds to a doubling of the c-parameter due to a global reverse rotation of TiO6 octahedra (similar to 15 degrees) around the c axis (see figure). As observed in La5Ti4O15 [4] and BaLa4Ti4O15 [5], this cooperative rotation of all the octahedra is Sr2+ cations with a [6+3] coordination simplest way to provide ation without any distortion of these octahedra. Due to the ionic radius of Sr2+, such a coordination is better suited than the [12] coordination.