Constitution of the Sr–Ni–O system

The constitution of the Sr–Ni–O system was studied experimentally for the first time. Samples were prepared either from SrCO3 and NiO or from Sr(NO3)2 and Ni(NO3)2·6H2O and characterized by high-temperature X-ray powder diffraction, scanning electron microscopy, thermogravimetric and differential thermal analyses. In the SrO–NiO quasibinary system an eutectic reaction: liquid⇌SrO+NiO was found to occur at 1396±5 °C, while the homogeneity range of terminal solid solutions is negligible. Thermodynamic calculations using the regular solution model for the liquid and rocksalt-type phases were employed to predict liquidus and solidus curves. Three ternary compounds, SrNiO2.5, Sr5Ni4O11, and Sr9Ni7O21 were observed in the samples prepared from nitrate solutions, but only Sr9Ni7O21 was proved to be thermodynamically stable in air up to 1030±6 °C. When heating in air, SrNiO2.5 and Sr5Ni4O11 were found to transform irreversibly into a mixture of Sr9Ni7O21 and NiO. Isothermal section of the SrO–NiO–O subsystem, which represents phase equilibria at 950–1030 °C as well as an isobaric section of the Sr–Ni–O system in air were constructed.