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

Subsolidus phase relations in Na2O–Fe2O3–Sb2Ox system (excluding Na-rich and Sb-rich corners) were studied using powder X-ray diffraction. Samples were prepared by conventional solid-state reactions at 980–1030 °C followed by quenching. Sb substitution for Fe stabilizes the low-temperature rhombohedral α form of NaFeO2 and enhances ionic conductivity: σ(300 °C) = 0.5 S/m, Ea = 0.38(3) eV, te < 0.01 for Na0.8Fe0.9Sb0.1O2 ceramics. Besides known orthorhombic Na2Fe3SbO8, three new compounds have been identified: trigonal Na4FeSbO6, a superlattice of α-NaFeO2 type, a = 5.4217(7) Å, c = 16.2715(1) Å, possible space group P3112; orthorhombic Na2FeSbO5, possibly related to brownmillerite, Pbcn, a = 10.8965(13) Å, b = 15.7178(13) Å, c = 5.3253(4) Å, and one more phase with empirical formula Na4Fe3SbO9, whose pattern could not be indexed. Ion-exchange reactions lead to a delafossite-type superlattice Ag3(NaFeSb)O6 (a = 5.4503(12) Å, c = 18.7747(20) Å, possible space group P3112).

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