| |
[摘要]：Chemical doping of 15 atom % Mn by Ti in 2H-BaMnO3 leads to two new polytypes, 4H- and 9R-BaMn0.85Ti0.15O3-delta , depending on the oxygen content. The 4H polytype (a = 5.71355(4) and c = 9.41230(8) A, space group P6(3)/mmc) was prepared in an inert atmosphere and forms face sharing dimers linked by corners based on a stacking sequence (he)(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) angstrom, space group R (3) over barm) with a (hhc)(3) stacking sequence; Mn4 ions are exclusively located on the central B-site (M1) of the face sharing trimers linked by corners with the oxygen vacancies located on the h-Ba03 layers leading to the formula Ba(Mn-0.33)m(1)(Mn0.52Ti0.15)M2O2.93(1). The 9R polytype is an antiferromagnetic semiconductor with a Neel temperature of 120 K and a relative permittivity of similar to 22. |
|
