Conditions Extrêmes et Matériaux : Haute Température et Irradiation

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Weiwei Cao, Xiaoyan Yang, Cécile Genevois, Mathieu Allix, Xiaojun Kuang, 'Extended B-Site Vacancy Content Range and Cation Ordering in Twinned Hexagonal Perovskites Ba8Cr4–xTa4+0.6xO24', Inorg. Chem. 60 3282-3290 (2021) doi:10.1021/acs.inorgchem.0c03707

Electrostatic repulsion between cations in face-sharing octahedral (FSO) sites is a key factor controlling the hexagonal perovskite structure formation. Ba2CrTaO6 has been known for more than 25 years as a line phase adopting an 8-layer twinned hexagonal perovskite structure with chromium atoms in the FSO dimers forming semimetal Cr-Cr bonding, which decreases the FSO B-B electrostatic repulsion. In this study, the aliovalent substitution of Ta5+ for Cr3+ in Ba2CrTaO6 led to an 8-layer twinned hexagonal solid solution Ba8Cr4-xTa4+0.6xO24 (x =0.0-3.0) containing FSO B-site vacancies. In this solid solution, the FSO B site occupancy varies from full occupation to 15% vacancies, in great contrast with the analogue Ba8Ga4-xTa4+0.6xO24 (1.8 ≤ x ≤ 3.2), in which Ga-rich compositions close to the B-site fully-occupied end member Ba2GaTaO6 do not form 8-layer twinned hexagonal perovskite phases. 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, with expanded a and b axes by compared to the simple 8-layer hexagonal perovskite structure owing to the partial 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 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 FSO B-B and B-O-B bonding and B-site vacancies. This work provides an effective way of combining B-B and B-OB bonding and vacancy creation as well as the cationic ordering in FSO sites to reduce electrostatic repulsion which could further enable the stabilization of new hexagonal perovskite compounds.