Synthesis and structure of Sr2MnGaO5+3 brownmillerites with variable oxygen content

The oxyen content in Sr2MnGaO5+delta brownmillerite can be smoothly varied in the -0.03 less than or equal to delta less than or equal to 0.13 and 0.41 less than or equal to delta less than or equal to 0.505 ranges by annealings at 415 degreesC at partial oxygen pressure ranging from 10(-5) atm to 1 atm. Between delta = 0.13 and delta = 0.41 the mixture of the reduced and oxidized phases is formed. The structures of some representative members of Sr2MnGaO5+delta series were studied using X-ray powder diffraction and electron diffraction. The increase of delta is accompanied by gradual decrease of orthorhombic distortion and symmetry change from Imma (0.03 less than or equal to delta less than or equal to 0.13, a approximate to c approximate to root2a(p), b approximate to 4a(p)) to Bmmm (0.41 less than or equal to delta less than or equal to 0.46, a approximate to c approximate to root2a(p), b approximate to 2a(p)) and to tetragonal P4/mmm (delta = 0.505, a approximate to a(p), c approximate to 2a(p)). Oxygen insertion in (GaO1+delta) layers results in the increase of Mn oxidation state from +3 to +4 and the compression of Mn-O apical bonds due to the suppression of the Jahn-Teller distortion of MnO6 octahedra. For delta = 0.41 the ordering of oxygen atoms was observed by electron diffraction, which breaks the symmetry down to monoclinic P2/m (a = 10.813 Angstrom, b = 8.019 Angstrom, c = 8.486 Angstrom, beta = 108.57degrees). The trends in variation of cell parameters with oxygen index delta are discussed taking into account major structural changes occurring upon oxidation. (C) 2003 Editions scientifiques et medicales Elsevier SAS. All rights reserved.