Phase equilibrium diagrams have been constructed from experimental data for the systems Sb2O4–NaSbO3, Sb2O4–KSbO3, and Sb2O4–NaSbO3–NaF. The system Sb2O4–NaSbO3 contains only an intermediate pyrochlore type solid solution with a maximum melting point of 1490 °C at a Na:Sb atom ratio of 3:5. The Sb2O4–KSbO3 system contains in addition to the pyrochlore phase a compound 3K2O • 5Sb2O5 which melts congruently at about 1450 °C and two polymorphs of K2O • 2Sb2O5. The low temperature form of K2O • 2Sb2O5 was found to be monoclinic P21/c with a = 7.178, b = 13.378, c = 11.985 A, β = 124°10′. The melting point of Sb2O4 was found to be 1350 ± 5 °C and NaSbO3 and KSbO3 both melt congruently at 1555 ± 5 °C and 1410 ± 5 °C respectively. The previously reported cubic form of KSbO3 was found to be a K+ deficient phase stabilized by reaction with atmospheric moisture. A similar cubic phase which appears to be a good Na+ ion conductor can be synthesized in the ternary system NaSbO3–Sb2O4–NaF.
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Waring et al. (1976) studied this question.