The oxygen content in Sr2MnGaO5+δ brownmillerite can be smoothly varied in the -0.03 ≤δ ≤ 0.13 and 0.41 ≤ δ < 0.505 ranges by annealings at 415 °C at partial oxygen pressure ranging from 10-5 atm to 1 atm. Between δ = 0.13 and δ = 0.41 the mixture of the reduced and oxidized phases is formed. The structures of some representative members of Sr2MnGaO5+δ series were studied using X-ray powder diffraction and electron diffraction. The increase of δ is accompanied by gradual decrease of orthorhombic distortion and symmetry change from Imma (0.03 ≤ δ ≤ 0.13, a ≈ c ≈ √2ap, b ≈ 4ap) to Bmmm (0.41 ≤ δ ≤ 0.46, a ≈ c ≈ √2ap, b ≈ 2ap) and to tetragonal P4/mmm (δ = 0.505, a ≈ ap, c ≈ 2ap). Oxygen insertion in (GaO1+δ) 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 δ = 0.41 the ordering of oxygen atoms was observed by electron diffraction, which breaks the symmetry down to monoclinic P2/m (a = 10.813 Å, b = 8.019 Å, c = 8.486 Å, β = 108.57°). The trends in variation of cell parameters with oxygen index δ are discussed taking into account major structural changes occurring upon oxidation.
- Crystal structure
- Oxygen non-stoichiometry