文摘
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, ag align=center border=0 SRC=/images/glyphs/BQ4.GIF>cg align=center border=0 SRC=/images/glyphs/BQ4.GIF>√2ap, bg align=center border=0 SRC=/images/glyphs/BQ4.GIF>4ap) to Bmmm (0.41≤δ≤0.46, ag align=center border=0 SRC=/images/glyphs/BQ4.GIF>cg align=center border=0 SRC=/images/glyphs/BQ4.GIF>√2ap, bg align=center border=0 SRC=/images/glyphs/BQ4.GIF>2ap) and to tetragonal P4/mmm (δ=0.505, ag align=center border=0 SRC=/images/glyphs/BQ4.GIF>ap, cg align=center border=0 SRC=/images/glyphs/BQ4.GIF>2ap). Oxygen insertion in (GaO1+δ) layers results in the increase of Mn oxidation state from +3 to +4 and the compression of Mng align=center border=0 SRC=/images/glyphs/BO7.GIF>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.
