Cation-deficient perovskite-related compounds (Ba,La)nTin–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)nTin–1O3n (n = 4, 5, 6) compounds have been re-examined using Rietveld full profile analysis of neutron diffraction patterns. La4Ti3O12 (n = 4) crystallizes with the R3 space group and the refined unit-cell parameters a = 5.5509(1) Å, c = 26.1780(2) Å, and Z = 3 (Rp = 8.48, Rwp = 9.08, Rbragg = 2.32). The Ba/La ordering in BaLa4Ti4O15 (n = 5) and Ba2La4Ti5O18 (n = 6) structures has been clarified. These structures can be described as consisting of perovskite-like slabs, n = 4, 5, and 6 corner-sharing TiO6 octahedra thick, respectively, separated by layers of vacant octahedra. From n = 4 to n = 6, with increasing amount of Ba atoms, the distortion of the mixed AO3 layers, i.e., the rotation of TiO6 octahedra around the c axis, decreases. The coordination of the A cation changes from [6 + 3] to [12].