Structure and reactivity with oxygen of Pr2NiO4+δ: an in situ synchrotron X-ray powder diffraction study

Abstract

The promising SOFC cathode material Pr₂NiO_4.22 has been studied in situ under a pure oxygen atmosphere from 25 to 950 °C by high resolution synchrotron X-ray powder diffraction. At room temperature (RT) δ = 0.22(1), the average crystal structure turns out to be monoclinic. The subtle monoclinic distortion (γ = 90.066(1)° at RT), retained up to 460 °C, is interpreted in terms of specific tilt schemes of the NiO₆ octahedra. It is also shown that Pr₂NiO_4.22 is incommensurately structurally modulated already at room temperature, in the same manner as the homologous cobaltate La₂CoO_4.14. The phase transition to the High Temperature Tetragonal (HTT) phase was completed at 480 °C without any evidence for the Low Temperature Orthorhombic (LTO) phase allowing clarifying the phase diagram of this K₂NiF₄-type ternary oxide. Moreover, it turns out that above 800 °C, the HTT phase transforms reversibly into two coexisting isomorphous tetragonal phases. The incommensurate modulation subsists up to 950 °C, although modified concomitantly with the two abovementioned phase transformations. In addition, the role of kinetics on the decomposition process is highlighted through thermo-gravimetric analyses.