Phase Equilibria in the La<sub>2</sub>O<sub>3</sub>-(Ni/Со)O-Sb<sub>2</sub>O<sub>5</sub> Systems in the Subsolidus Region

Subsolidus phase equilibria in the La2O3–(Ni/Со)O–Sb2O5 systems have been studied. A previously unknown compound La4Sb2O11 was found in the system La2O3–Sb2O5. La4Sb2O11 has been shown to be decomposed at a temperature of 1060°C to form La3SbO7 and LaSbO4. Two ternary oxides LaNi2SbO6 and La2NiSb2O9 were found in the La2O3–NiO–Sb2O5 system for the first time. These new compounds are stable and do not undergo polymorphic transformations throughout the studied temperature range (25–1350°C). The existence of previously known triple oxides La3Ni2SbO9 and LaNi1/3Sb5/3O6 with structures of perovskite and rosiaite, respectively, has also been confirmed. Two more new compounds LaCo2SbO6 and La2CoSb2O9 are formed in the La2O3–CoO–Sb2O5 system along with previously known compounds with the structures of perovskite La3Co2SbO9, rosiaite LaCo1/3Sb5/3O6 and rhombohedral pyrochlore La3Co2Sb3O14. These compounds are isostructural to those found in the nickel oxide system. The La2CoSb2O9 compound, unlike similar nickel compound, decomposes at a temperature of 990°C. For LaCo2SbO6, no thermal effects on DSC curves associated with polymorphic transitions or melting were detected up to 1350°C. Analysis of the optical diffuse reflection spectra of the newly synthesized phases LaNi2SbO6, La2NiSb2O9, LaCo2SbO6 and La2CoSb2O9 showed that nickel and cobalt in them are in the oxidation state of 2+. Isothermal sections of La2O3–(Ni/Co)O–Sb2O5 systems at 1050°C have been constructed.