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

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 LaNi1/3Sb5/3O6, 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 LaCo1/3Sb5/3O6, 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.