Subsolidus phase relations, crystal chemistry and cation-transport properties of sodium iron antimony oxides

摘要

Subsolidus phase relations in Na_2O-Fe_2O_3-Sb_2O_x system (excluding Na-rich and Sb-rich comers) were studied using powder X-ray diffraction. Samples were prepared by conventional solid-state reactions at 980-1030 deg C followed by quenching. Sb substitution for Fe stabilizes the low-temperature rhombohedral a form of NaFeO_2 and enhances ionic conductivity: sigma(300 deg C) = 0.5 S/m, E_a = 0.38(3) eV, t_e < 0.01 for Na_(0.8)Fe_(0.9)Sb_(0.1)O_2 ceramics. Besides known orthorhornbic Na_2Fe_3SbO_8, three new compounds have been identified: trigonal Na_4FeSbO_6, a superlattice of alpha-NaFeO_2 type, a = 5.4217(7) A, c = 16.2715(1) A, possible space group P3_1 12, orthorhombic Na_2FeSbO_5, possibly related to brownmillerite, Pbcn, a = 10.8965(13) A, b = 15.7178(13) A, c = 5.3253(4) A, and one more phase with empirical formula Na_4Fe_3SbO_9, whose pattern could not be indexed. Ion-exchange reactions lead to a delafossite-type superlattice Ag_3(NaFeSb)O_6(a = 5.4503(12) A, c = 18.7747(20) A, possible space group P3_1 12).