Phase Equilibria and Crystal Chemistry of Rubidium Niobates and Rubidium Tantalates

The phase equilibria relations and crystal chemistry of portions of the Rb₂O-Nb₂O₅ and Rb₂O-Ta₂O₅ systems were investigated for structures potentially useful as ionic conductors. A hexagonal tungsten bronze-type (HTB) structure was found in both systems as well as three hexagonal phases with mixed HTB-pyrochlore type structures. Ion exchange experiments between various alkali ions are described for several phases. Unit cell dimensions and x-ray diffraction powder patterns are reported.

Phase Equilibria and Crystal Growth in the Alkali Antimonate Systems Sb2O4-NaSbO3/ Sb2O4-KSbO3/, and Sb2O4-NaSbO3-NaF

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.

Temperature-Pressure Phase Relationships in Niobium Pentoxide

A pressure-temperature (P-T) section of the phase equilibrium diagram for Nb2O5 has been determined. Four single phase regions, H-Nb2O5, B-Nb2O5, and two L-Nb2O5 areas, were characterized by x-ray powder diffraction data. The superstructure of L-Nb2O5 was indexed on the basis of similarities to the known superstructures of L-Ta2O5 and confirmed with single crystal electron diffraction data. The correct unit cell is orthorhombic with a = 6.168, b = 29.312, c = 3.938 Å, with the b axis 8 times the subcell (8 × 3.664 Å).