First 14-Layer Twinned Hexagonal Perovskite Ba14Mn1.75Ta10.5O42: Atomic-Scale Imaging of Cation Ordering

Formation of hexagonal perovskite with mixed cubic and hexagonal stacking of AO3 layers becomes more and more difficult when the number of layers in the stacking repeating unit increases. So far, the highest number of layers reported for twinned hexagonal perovskite is 12, with alternative 5 consecutive cubic layers and one hexagonal layer in the (h)2 sequence. Here, we present the unexpected formation of a 14-layer twinned hexagonal perovskite with a stacking sequence (h)2 for the BaO3 layers on the Ba14Mn1.75Ta10.5O42 (Ba8MnTa6O24) composition, the first example of twinned hexagonal perovskite with a periodicity exceeding 12-layers. The B-cation and vacancy distributions are characterized by multiple efficient and complementary techniques including neutron and synchrotron powder diffraction, scanning transmission electron microscopy-high angle annular dark field (STEM-HAADF) imaging, and electron energy loss spectroscopy (EELS) and X-ray energy dispersive spectroscopy (EDS) elemental mapping....