Crystal chemistry and redox behaviour of perovskite-related bismuth strontium calcium oxides

Abstract

The synthesis and characterization of two solid solutions in the Bi–Sr–Ca–O system are reported. They have the same cationic stoichiometry but different oxygen content. The so-called oxidized solid solution (Sr_2–xCa_x)(Bi_1.4Ca_0.6)O₆ exists for 0≤x≤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) Å, β_o=89.946(5) °, and the space group is P2₁ /n. It shows a slightly distorted perovskite structure with parameters √2a_p ×√2a_p ×2a_p and two crystallographically independent B positions with occupancies 95% Bi/5% Ca (site 1) and 45% Bi/65% Ca (site 2) ordered in a 3D NaCl-type arrangement. The reduced solid solution, (Sr_2–xCa_x)(Bi_1.4Ca_0.6)O_6–δ(δ=0.79), exists for the same cationic range and can be obtained from the oxidized solid solution by reduction in air or oxygen at 800 °C. This process is reversible: the reduced solid solution leads to the oxidized one by treatment in oxygen at 650 °C. At lower temperatures other oxidation–reduction reactions take place without apparent modification of the structure of the starting 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=5.9097(3), c_r=20.0787(9) Å, β_r=101.747(4) ° in space group P2₁ /c; 63.6% of bismuth is in the +3 valence state and 36.4% is in the +5 valence state.