CRYSTAL-CHEMISTRY AND REDOX BEHAVIOR OF PEROVSKITE-RELATED BISMUTH STRONTIUM CALCIUM OXIDES

The synthesis and characterization of two solid solutions in the Bi-Sr -Ca-O system are reported. They have the same cationic stoichiometry b ut different oxygen content. The so-called oxidized solid solution (Sr 2-xCax)(Bi1.4Ca0.6)O-6 exists for 0 less than or equal to x less than or equal to 0.5. The cell parameters for the member with x = 0 are a(o ) 5.89228(9),b(o) = 5.98928(8), c(o) = 8.38336(14) Angstrom, beta(o) = 89.946(5)degrees, and the space group is P2(1)/n. It shows a slightly distorted perovskite structure with parameters root 2a(p) x root 2a(p ) X 2a(p) and two crystallographically independent B positions with oc cupancies 95% Bi/5% Ca (site 1) and 45% Bi/65% Ca (site 2) ordered in a 3D NaCl-type arrangement. The reduced solid solution, (Sr2-xCax)(Bi1 .4Ca0.6)O6-delta (delta = 0.79), exists for the same cationic range an d can be obtained from the oxidized solid solution by reduction in air or oxygen at 800 degrees C. This process is reversible: the reduced s olid solution leads to the oxidized one by treatment in oxygen at 650 degrees C. At lower temperatures other oxidation-reduction reactions t ake place without apparent modification of the structure of the starti ng phases showing oxygen non-stoichiometry. The reduced solid solution shows a perovskite-related structure with cell parameters for x = 0: a(r) = 11.2396(5), b(r) = 59097(3), c(r) = 20.0787(9) Angstrom, beta(r ), = 101.747(4)degrees in space group P2(1)/c; 63.6% of bismuth is in the + 3 valence state and 36.4% is in the + 5 valence stare.