Composition-Structure-Property Relationships of 6H-and 12R-Type Hexagonal Ba(Mn,Ti)O3-δ Perovskites

The crystal structures of 6H-type BaMn0.15Ti0.85O3, BaMn1/4Ti3/4O2.95, and BaMn1/2Ti1/2O2.84 and 12R-type BaMn2/3Ti1/3O3 have been established by a combination of X-ray, neutron, and electron diffraction, and high-resolution electron microscopy. The 6H-type structure (space group P6(3)/mmc) can be described by a stacking sequence (hcc)(2) along the c-axis with any anion deficiency located exclusively in the h-BaO3 layers. Ti atoms display a strong preference for the corner-shared octahedral site, whereas both Mn and Ti are distributed over the octahedral sites in the face sharing dimers. The 12R-type structure (space group R (3) over barm) can be described by a stacking sequence (hhcc)(3). Ti atoms again display a strong preference for the isolated corner-sharing octahedral site, whereas Mn atoms occupy the central site of the face-sharing trimers. The electrical properties have been characterized by impedance spectroscopy and reveal the fully oxidized compounds to be electrical insulators with relative permittivity values of similar to 45-55 at 300 K. The oxygen-deficient compounds are semiconductors, which is attributed to the presence of mixed Mn3+ and Mn4+ ions on the B-site sublattice. Antiferromagnetic (AFM) interactions occur within the face-sharing units of the respective structures. The AFM interactions inside the dimers (6H-type) and trimers (12R-type) become stronger with increasing Mn content and result in an increase in the magnitude of the Curie-Weiss constant.