STRUCTURE DETERMINATIONS FOR CA3TI2O7, CA4TI3O10, CA3.6SR0.4TI3O10 AND A REFINEMENT OF SR3TI2O7

The structures of the orthorhombic Ruddlesden-Popper (A(n+1)B(n)X3n+1) phases Ca3Ti2O7 (n = 2) [M(r) = 328.04, Ccm2(1), a = 5.4172 (1), b = 19.5169 (4), c = 5.4234 (1) angstrom, V = 573.40 angstrom 3, Z = 4, D(x) = 3.80 g cm-3, R(B) = 1.37%], Ca4Ti3O10 (n = 3) [M(r) = 464.02, Pcab, a = 5.4083 (1), b = 27.1432 (4), c = 5.4337 (1) angstrom, V = 797.66 angstrom 3, Z = 4, D(x) = 3.86 g cm-3, R(B) = 1.53%] and Ca3.6Sr0.4Ti3O10 (n = 3) [M(r) = 483.04, Pcab, a = 5.4409, b = 27.2727 (7), c = 5.4415 (2) angstrom, V = 807.45 angstrom 3, Z = 4, D(x) = 3.97 g cm-3, R(B) = 1.55%] have been determined, and that of tetragonal Sr3Ti2O7 [M(r) = 470.66, 14/mmm, a = 3.9026 (1), c = 20.3716 (4) angstrom, V = 310.27 angstrom 3, Z = 2, D(x) = 5.55 g cm-3, R(B) = 1.54%] refined from neutron powder diffraction data at lambda = 1.893 angstrom. They consist of coherent intergrowths of perovskite (CaTiO3) blocks, n TiO6 octahedra thick, with single layers of CaO having a distorted NaCl configuration. TiO6 octahedra are titled and distorted in a very similar fashion to those in CaTiO3 (n = infinity). This fact was used to determine the space groups of the layered structures. Convergent-beam electron diffraction patterns are best matched by calculations in the above space groups which are thus confirmed. Octahedral tilt angles increase slightly in the sequence n = 2, 3, infinity. Strontium addition reduces the octahedral tilt angles because of preferential substitution of Sr on the Ca sites within the perovskite blocks of Ca4Ti3O10. The algorithm used to produce starting models for structure refinements is thought to be generally applicable to Ruddlesden-Popper and possibly other layered perovskite structures. It furnishes the predictions: (a) all n-even compounds in the Ca(n+1)Ti(n)O3n+1 series will have space group Ccm2(1), (b) all n-odd compounds in this series will have space group Pcab, (c) all A(n+1)B(n)X3n+1 series for which the n = infinity end member (ABX3) is isostructural with CaTiO3 will be isostructural with the compounds reported above (e.g. Ca(n+1)Zr(n)O3n+1).