New barium manganese titanates prepared under reducing conditions

We report the synthesis and initial characterization of BaMnTi5O12, the first example of an oxide displaying an interesting tripled hollandite-type structure. Shiny black crystals which crystallize in the monoclinic space group, P2/n, with a = 9.921(3) Angstrom, b = 8.8716(22) Angstrom, c = 10.309(3) Angstrom, and beta = 91.24(3)degrees, were grown from a borate-based melt in vacuo. The structure contains layers made up of edge-shared pairs of TiO6, octahedra forming a hollandite framework. The Ba2+ ions occupy large high coordination number channel sites. Oxidation states of the transition metal ions are such that a charge balance of one half barium ion per three framework units with an ideal stoichiometry Ba0.5M3O6, produces the tripling. In addition, crystals of Ba2MnTi5O13, monoclinic, space group C2/m, a = 15.075(1) Angstrom, b = 3.9470(2) Angstrom, c = 9.0985(5) Angstrom, and beta = 98.23(1)degrees, which is isostructural with the mineral jeppeite, K2Ti6O13, were also grown. (C) 2002 Editions scientifiques et medicales Elsevier SAS. All tights reserved.