Extended B-Site Vacancy Content Range and Cation Ordering in Twinned Hexagonal Perovskites Ba₈Cr_4-Ta_4+0.6O₂₄

The new 8-layer twinned hexagonal solid solution Ba₈Cr_4-Ta_4+0.6O₂₄ (x = 0.0-3.0) was isolated through the aliovalent substitution of Ta⁵⁺ for Cr³⁺ in Ba₂CrTaO₆, showing the widest B-site vacancy content range among the 8-layer twinned hexagonal perovskites. Ba₈Cr_4-Ta_4+0.6O₂₄ forms a simple 8-layer hexagonal perovskite structure within 0.0 ≤ x < 2.4 and a tripled 8-layer hexagonal perovskite superstructure within 2.4 ≤ x ≤ 3.0. The latter shows expanded a and b axes by $$ times in comparison to the simple 8-layer hexagonal perovskite structure owing to the partial face-sharing octahedral (FSO) B cation ordering along the ab plane. The B-cation and vacancy distributions in the tripled superstructure were characterized by neutron and X-ray powder diffraction and further confirmed by a scanning transmission electron microscopy-high angle annular dark field imaging and intensity profile analysis. The formation of 8-layer twinned hexagonal perovskites Ba₈Cr_4-Ta_4+0.6O₂₄ in an extended solid solution range can be attributed to the presence of both covalent B-B and B-O-B bonding and B-site vacancies in the FSO sites. This work provides an effective way of combining covalent B-B and B-O-B bonding and vacancy creation as well as the cationic ordering in the FSO sites to reduce electrostatic repulsion, which could further enable the stabilization of new hexagonal perovskite compounds.