Subsolidus Phase Equilibria in the La2O3–(Ni/Со)O–Sb2O5 Systems

Abstract

Subsolidus phase equilibria in the La₂O₃–(Ni/Со)O–Sb₂O₅ systems have been studied. A previously unknown compound La₄Sb₂O₁₁ was found to exist in the La₂O₃–Sb₂O₅ system. La₄Sb₂O₁₁ has been shown to decompose at 1060°C to form La₃SbO₇ and LaSbO₄. Two ternary oxides (LaNi₂SbO₆ and La₂NiSb₂O₉) have been discovered in the La₂O₃–NiO–Sb₂O₅ system. These new compounds are stable and do not undergo polymorphic transformations throughout the range of temperatures studied (25–1350°C). The existence of previously known ternary oxides, namely perovskite La₃Ni₂SbO₉ and rosiaite LaNi_1/3Sb_5/3O₆, has been verified. In the La₂O₃–CoO–Sb₂O₅ system, two new compounds (LaCo₂SbO₆ and La₂CoSb₂O₉) have been found along with previously known perovskite La₃Co₂SbO₉, rosiaite LaCo_1/3Sb_5/3O₆, and rhombohedral pyrochlore La₃Co₂Sb₃O₁₄. These new compounds are isostructural to those found in the nickel oxide system. La₂CoSb₂O₉, unlike its nickel analogue, decomposes at 990°C. For LaCo₂SbO₆, no thermal events associated with polymorphic transitions or melting have been detected on DSC curves up to 1350°C. An inspection of diffuse reflectance spectra of the newly synthesized LaNi₂SbO₆, La₂NiSb₂O₉, LaCo₂SbO₆, and La₂CoSb₂O₉ phases showed that the oxidation state of nickel and cobalt in them is 2+. The 1050°C isothermal sections of La₂O₃–(Ni/Co)O–Sb₂O₅ systems have been constructed.