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) Å, b = 8.8716(22) Å, c = 10.309(3) Å, and β = 91.24(3)°, 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 Ba_0.5M₃O₆, produces the tripling. In addition, crystals of Ba₂MnTi₅O₁₃, monoclinic, space group C2/m, a = 15.075(1) Å, b = 3.9470(2) Å, c = 9.0985(5) Å, and β = 98.23(1)°, which is isostructural with the mineral jeppeite, K₂Ti₆O₁₃, were also grown.