Extended B-Site Vacancy Content Range and Cation Ordering in Twinned Hexagonal Perovskites Ba 8 Cr 4– x Ta 4+0.6 x O 24
Résumé
The new 8-layer twinned hexagonal solid solution Ba 8 Cr 4−x Ta 4+0.6x O 24 (x = 0.0−3.0) was isolated through the aliovalent substitution of Ta 5+ for Cr 3+ in Ba 2 CrTaO 6 , showing the widest B-site vacancy content range among the 8-layer twinned hexagonal perovskites. Ba 8 Cr 4−x Ta 4+0.6x O 24 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 3 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 Ba 8 Cr 4−x Ta 4+0.6x O 24 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.
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