Structural Polymorphism of Mn-Doped BaTiO3

The crystal structure of BaTi1−xMnxO3 (0 ≤ x ≤ 0.5) has been determined by means of neutron powder diffraction. Upon Mn doping, the BaTi1−xMnxO3 system undergoes structural transformations from a polar tetragonal structure with space group P4mm to a non-polar 6H-type hexagonal structure with space group P63/mmc at x > 0.01, and then to a non-polar 12R-type rhombohedral structure with space group R-3m at x > 0.12. For the ferroelectric tetragonal phase, Mn doping leads to a reduction of the spontaneous polarization and the Curie temperature. In the 6H structure, Ti atoms display a strong preference for the corner-sharing octahedral sites, whereas both Ti and Mn randomly occupy the octahedral sites in the face-sharing dimers. In the 12R-structure, Ti atoms also have a strong preference for the corner-sharing octahedral sites, whereas Mn atoms occupy the octahedral sites at the centers of the face-sharing octahedral trimers. Both Ti and Mn atoms are distributed over the octahedral sites at the borders of the trimers. The absence of long-range magnetic order in the 6H-type and 12R-type phases was observed, which is due to the presence of the non-magnetic Ti ions at the centers of the corner-sharing octahedra connecting the face-sharing dimers (6H-type) and trimers (12R-type), breaking the magnetic interaction between the dimers/trimers and isolating them from each other.