Crystal chemistry and redox behaviour of antimony strontium calcium perovskites

The compound Sr2Sb1.4Ca0.6O6 and their reduced forms SP2Sb1.4Ca0.6O5.17 and Sr2Sb1.4Ca0.6O4.84 have been prepared and characterized by powder X-ray di ffraction, electron diffraction, iodometric analyses and thermogravimetric analysis. The three phases with different oxygen stoichiometries are struct urally related to the perovskite and show symmetry distortions from the ide al cubic structure (with cell parameter a(p)). The crystal structure of Sr2 Sb1.4Ca0.6O6 may be refined by the Rietveld method from powder X-ray diffra ction data using the space group P2(1)/n, and the cell parameters a=5.776(2 ), b=5.7837(2), c=8.1718(3) Angstrom, beta =90.039(3)degrees 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 r educed phases, Sr2Sb1.4Ca0.6O5.17 and Sr2Sb1.4Ca0.6O4.84, are obtained from Sr2Sb1.4Ca0.6O6 by treatment in Ar or Ar/H-2 at 650 degreesC and show resp ectively the unit cell parameters a approximate tob approximate to5.9 Angst rom, c approximate to8.4 Angstrom (with possible space groups I2/m or Immm) , and a=8.4 Angstrom, b=6.0 Angstrom, c=24.2 Angstrom, beta =125 degrees (s pace group C2/c). The reduction process is reversible, and the initial oxid ized phase can be obtained from the reduced ones by thermal treatment in ox ygen or air, at temperatures close to 500 degreesC. (C) 2000 Elsevier Scien ce Ltd. All rights reserved.