Phase Equilibria and Crystal Growth in the Alkali Antimonate Systems Sb(2)O(4)–NaSbO(3/) Sb(2)O(4)–KSbO(3/), and Sb(2)O(4)–NaSbO(3)–NaF

Phase equilibrium diagrams have been constructed from experimental data for the systems Sb(2)O(4)–NaSbO(3), Sb(2)O(4)–KSbO(3), and Sb(2)O(4)–NaSbO(3)–NaF. The system Sb(2)O(4)–NaSbO(3) 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 Sb(2)O(4)–KSbO(3) system contains in addition to the pyrochlore phase a compound 3K(2)O • 5Sb(2)O(5) which melts congruently at about 1450 °C and two polymorphs of K(2)O • 2Sb(2)O(5). The low temperature form of K(2)O • 2Sb(2)O(5) was found to be monoclinic P2(1)/c with a = 7.178, b = 13.378, c = 11.985 A, β = 124°10′. The melting point of Sb(2)O(4) was found to be 1350 ± 5 °C and NaSbO(3) and KSbO(3) both melt congruently at 1555 ± 5 °C and 1410 ± 5 °C respectively. The previously reported cubic form of KSbO(3) 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 NaSbO(3)–Sb(2)O(4)–NaF.