Order–disorder character and twinning in the structure of a new synthetic titanosilicate: (Ba,Sr)4Ti6Si4O24·H2O

Abstract

Prismatic crystals of the title compound, up to 100 microm long and {001} twinned by metric merohedry, were obtained as a side-product of a hydrothermal run devoted to synthesizing the heterophyllosilicate lamprophyllite. Single-crystal X-ray diffraction data were collected on a Bruker-AXS Smart Apex diffractometer from a crystal with approximate composition (Ba(0.80)Sr(0.20))(4)Ti6Si4O24 x H2O. The structure was solved and refined as a disordered structure in the space group Cmmm [a(o) = 5.906 (2), b(o) = 20.618 (8), c(o) = 16.719 (6) A, R = 0.089 for 682 reflections with I(o) > 2sigma(I(o))], and then deciphered by the order-disorder (OD) theory as an ordered structure and refined in the space group P2/c [a = 5.906, b = 16.719, c = 10.724 A, beta = 105.99 degrees , R = 0.083 for 1090 reflections with I(o) > 2sigma(I(o))]. The discussion based on the OD theory shows that the refined ordered structure corresponds to one (2M) of two maximum degree of order (MDO) polytypes. The structure of the second MDO polytype (4O) was modelled but not refined because it does not substantially occur in our sample. In the structure, infinite (001) ribbons of Ti octahedra elongated along the [100] direction are connected by (SiO3)4 four-membered rings, thus realising a new type of heteropolyhedral framework. The ribbons are three-octahedra wide and one-octahedron thick; they are formed by linking, via edge-sharing, rutile-type edge-sharing rows of octahedra. This ribbon represents a slice of the octahedral sheet that occurs in perrierite-(Ce), Ce4MgFe2Ti2O8(Si2O7)2. The (Ba,Sr) ions are hosted within two independent [100] narrow channels both delimited by four Ti octahedra and four Si tetrahedra. The disorder of H2O is discussed on the basis of a Raman spectrum.