Cation-deficient perovskite-related compounds (Ba,La)(n)Tin-1O3n (n=4, 5, and 6): a Rietveld refinement from neutron powder diffraction data

The crystal structures of the three basic members of the homologous series of (Ba,La)(n)Tin-1O3n (n = 4, 5, 6) compounds have been re-examined using R ietveld full profile analysis of neutron diffraction patterns. La4Ti3O12 (n = 4) crystallizes with the R(3) over bar space group and the refined unit- cell parameters n = 5.5509(1) Angstrom, c = 26.1780(2) Angstrom, and Z = 3 (R-p = 8.48, R-wp = 9.08, R-bragg = 2.32). The Ba/La ordering in BaLa4Ti4O1 5 (n = 5) and Ba2La4Ti5O18 (n = 6) structures has been clarified. These str uctures can be described as consisting of perovskite-like slabs, n = 4, 5, and 6 corner-sharing TiO6 octahedra thick, respectively, separated by layer s of vacant octahedra. From n = 4 to n = 6, with increasing amount of Ba at oms, the distortion of the mixed AO(3) layers, i.e., the rotation of TiO6 o ctahedra around the c axis, decreases. The coordination of the A cation cha nges from [6 + 3] to [12]. (C) 2000 Elsevier Science Ltd. All rights reserv ed.