Structure and microstructure of hexagonal Ba 3 Ti 2 RuO 9 by electron diffraction and microscopy

We have used electron microscopy and diffraction to refine the structure and investigate the microstructure of Ba 3 Ti 2 RuO 9 . The parent compound is hexagonal BaTiO 3 with the space group P 6 3 / mmc . Using convergent-beam electron diffraction (CBED) combined with electron-sensitive image plates we have found that the space group of Ba 3 Ti 2 RuO 9 is the non-centrosymmetric group P 6 3 mc at room temperature and at ∼ 110 K. This is consistent with the Ru and Ti atoms occupying alternate face-sharing octahedral sites in the 〈0001〉 direction. This maintains the c -glide, but breaks the mirror normal to the c axis and consequently removes the centre of symmetry. Using powder X-ray diffraction, we have measured the lattice parameters from polycrystalline samples to be a = 5.7056 ± 0.0005, c = 14.0093 ± 0.0015 Å at room temperature. Using high-resolution electron microscopy (HREM) we observed highly coherent, low-strain {10\bar 10} grain boundaries intersecting at 60 and 120°. From CBED we deduce that adjacent grains are identical but for the relative phase of the Ti and Ru atom ordering along the c axis. HREM also revealed occasional stacking faults, normal to the c -axis.