Abstract
The compound Sr2Sb1.4Ca0.6O6 and their reduced forms Sr2Sb1.4Ca0.6O5.17 and Sr2Sb1.4Ca0.6O4.84 have been prepared and characterized by powder X-ray diffraction, electron diffraction, iodometric analyses and thermogravimetric analysis. The three phases with different oxygen stoichiometries are structurally related to the perovskite and show symmetry distortions from the ideal cubic structure (with cell parameter ap). The crystal structure of Sr2Sb1.4Ca0.6O6 may be refined by the Rietveld method from powder X-ray diffraction data using the space group P21/n, and the cell parameters a=5.776(2), b=5.7837(2), c=8.1718(3) Å, β=90.039(3)° with the same structural model than for previously studied Sr2Bi1.4Ca0.6O6. It shows two crystallographically independent B positions with occupancies 100% Sb (site 1) and 40% Sb/60% Ca (site 2) ordered in a 3D NaCl-type arrangement. The reduced phases, Sr2Sb1.4Ca0.6O5.17 and Sr2Sb1.4Ca0.6O4.84, are obtained from Sr2Sb1.4Ca0.6O6 by treatment in Ar or Ar/H2 at 650°C and show respectively the unit cell parameters a≈b≈5.9 Å, c≈8.4 Å (with possible space groups I2/m or Immm), and a=8.4 Å, b=6.0 Å, c=24.2 Å, β=125° (space group C2/c). The reduction process is reversible, and the initial oxidized phase can be obtained from the reduced ones by thermal treatment in oxygen or air, at temperatures close to 500°C.
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