Composition−Structure−Property Relationships of 6H- and 12R-Type Hexagonal Ba(Mn,Ti)O_3−δPerovskites

The crystal structures of 6H-type BaMn 0.15 Ti 0.85 O 3, BaMn 1/4 Ti 3/4 O 2.95, and BaMn 1/2 Ti 1/2 O 2.84 and 12R-type BaMn 2/3 Ti 1/3 O 3 have been established by a combination of X-ray, neutron, and electron diffraction, and high-resolution electron microscopy. The 6H-type structure (space group P 6 3 / mmc ) can be described by a stacking sequence (hcc) 2 along the c -axis with any anion deficiency located exclusively in the h-BaO 3 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 3̅ m ) 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 ∼45−55 at 300 K. The oxygen-deficient compounds are semiconductors, which is attributed to the presence of mixed Mn 3+ and Mn 4+ 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.