Polytypism in the BaMn0.85Ti0.15O3−δ System (0.07≤δ≤0.34). Structural, Magnetic, and Electrical Characterization of the 9R-Polymorph

Chemical doping of 15 atom % Mn by Ti in 2H-BaMnO3 leads to two new polytypes, 4H- and 9R-BaMn0.85Ti0.15O3−δ, depending on the oxygen content. The 4H polytype (a = 5.71355(4) and c = 9.41230(8) A, space group P63/mmc) was prepared in an inert atmosphere and forms face sharing dimers linked by corners based on a stacking sequence (hc)2 where h and c refer to hexagonal and cubic BaO3 layers, respectively. Selected area electron diffraction patterns and high resolution electron microscopy reveal the coexistence of two types of crystals; the majority corresponds to an ordered (hc)2-4H polytype whereas a minority fraction shows a disordered microstructure. Such differences may be associated with nonhomogeneous cationic distribution within the crystals. Oxidation in air at 1523 K for 48 days leads to conversion to the 9R polytype (a = 5.68007(1) and c = 20.98208(6) A, space group R3m) with a (hhc)3 stacking sequence; Mn4+ ions are exclusively located on the central B-site (M1) of the face sharing trimers linke...