Thermochemistry of Sr ₂Ce _1-xPr _xO ₄ (x = 0, 0.2, 0.5, 0.8, and 1): Variable-temperature and -atmosphere in-situ and ex-situ powder x-ray diffraction studies and their physical properties

A novel family of metal oxides with a chemical formula of Sr ₂Ce _1-xPr _xO ₄ (x = 0, 0.2, 0.5, 0.8, and 1) was developed as mixed oxide ion and electronic conductors for solid oxide fuel cells (SOFCs). All of the investigated samples were synthesized by the ceramic method at 1000 °C in air and characterized by powder X-ray diffraction (PXRD), selected area electron diffraction (SAED), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and electrochemical impedance spectroscopy (EIS). Ex-situ PXRD reveals that the Sr ₂PbO ₄-type Sr ₂CeO ₄ decomposes readily into a mixture of perovskite-type SrCeO ₃ and rock-salt-type SrO at 1400 °C in air. Surprisingly, the decomposed products are converted back to the original Sr ₂PbO ₄-type Sr ₂CeO ₄ phase at 800 °C in air, as confirmed by in-situ PXRD. Thermal decomposition is highly suppressed in Sr ₂Ce _1-xPr _xO ₄ compounds for Pr > 0, suggesting that Pr improves the thermal stability of the compounds. Rietveld analysis of PXRD and SAED supported that both Pr and Ce ions are located on the 2a site in Pbam (space group no. 55). The electrical transport mechanism could be correlated to the reduction of Pr and/or Ce ions and subsequent loss of oxide ions at elevated temperatures, as shown by TGA and in-situ PXRD. Conductivity increases with Pr content in Sr ₂Ce _1-xPr _xO ₄. The highest total conductivity of 1.24×10 ^-1 S cm ^-1 was observed for Sr ₂Ce _0.2Pr _0.8O ₄ at 663 °C in air.