First 14-Layer Twinned Hexagonal Perovskite Ba₁₄Mn_1.75Ta_10.5O₄₂: 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 (ccccch)₂ sequence. Here, we present the unexpected formation of a 14-layer twinned hexagonal perovskite with a stacking sequence (cccccch)₂ for the BaO₃ layers on the Ba₁₄Mn_1.75Ta_10.5O₄₂ (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. Atomic-resolution STEM-HAADF imaging and EELS/EDS elemental mapping enables direct observation of high-spin d⁵ Mn²⁺ cation ordering in the d⁰ Ta⁵⁺ host, thus demonstrating the great potential of this technique for probing cation ordering and performing structure determination. Moreover, atomic mapping allows for the observation of local defect structure variants, which can be a powerful tool for future new material design. The large high-spin Mn²⁺ cation and Ta-vacancy pair formation in face-sharing octahedral sites play key roles on both the stabilization of this 14-layer twinned hexagonal perovskite structure and the Mn²⁺ ordering in the central corner-sharing octahedral (CSO) positions within the five-consecutive CSO layers. Compared with the 8-layer twinned Ba₈ZnTa₆O₂₄ material, the low quality factor in microwave frequency and enhanced ultraviolet and visible light absorption of Ba₁₄Mn_1.75Ta_10.5O₄₂ as well as the photocatalytic activity on water splitting are discussed in terms of the presence of high-spin Mn²⁺ cations in the structure.