Extended B-Site Vacancy Content Range and Cation Ordering in Twinned Hexagonal Perovskites Ba8Cr4–xTa4+0.6xO24

The new 8-layer twinned hexagonal solid solution Ba8Cr4–xTa4+0.6xO24 (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. Ba8Cr4–xTa4+0.6xO24 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 Ba8Cr4–xTa4+0.6xO24 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.