Electron diffraction and microscopy study of the structure and microstructure of the hexagonal perovskite Ba3Ti2MnO 9

This paper reports a structural and microstructural investigation of the hexagonal perovskite Ba₃Ti₂MnO₉ using electron microscopy and diffraction. Convergent-beam electron diffraction (CBED) revealed the structure has the non-centrosymmetric space group P6₃mc (186) at room temperature and at ∼ 110 K. Compared with the centrosymmetric parent structure BaTiO₃, with space group P6₃/mmc, this represents a break in mirror symmetry normal to the c axis. This implies the Ti and Mn atoms are ordered on alternate octahedral sites along the 〈0001〉 direction in Ba₃Ti₂MnO₉. Using high-resolution electron microscopy (HREM), we observed occasional 6H/12R interfaces on (0001) planes, however, no antiphase boundaries were observed, as were seen in Ba₃Ti₂RuO₉. Using powder X-ray Rietveld refinement we have measured the lattice parameters from polycrystalline samples to be a = 5.6880 ± 0.0005, c = 13.9223 ± 0.0015 Å at room temperature.