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article
Crystallographic and magnetic structures of Y₀.₈Sr₂.₂Mn₂GaO₈-ɗ : a new vacancy-ordered perovskite structure
Abstract
The anion-deficient perovskite Y0.8Sr2.2Mn2GaO8-delta (where delta similar to 0.1) has been synthesised and the crystal and magnetic structures determined by Rietveld analysis of neutron powder diffraction (NPD) data. The material has body-centred tetragonal symmetry (14/mmm, a = 7.6373(3) Angstrom and c = 15.6636(10) Angstrom) and consists of alternating layers of octahedral and tetrahedral polyhedra, the layers being perpendicular to [001]. The octahedral layers are preferentially occupied by manganese and the tetrahedral layers are a mixture of manganese and gallium. The precise cation distribution depends critically on preparative conditions. An unusual structural feature of these materials is the arrangement of oxygen vacancies in the tetrahedral layers: in the basic structure, isolated squares of corner-linked tetrahedra are formed instead of the chains that are observed in brownmillerite phases. Additional oxide ions in this layer probably allow the Mn ions to achieve distorted square pyramidal coordination. Low temperature NPD and magnetisation data indicate antiferromagnetic ordering below 100 K. (C) 2003 Elsevier Ltd. All rights reserved.