High-temperature redox chemistry of Pr0.5Sr1.5Cr0.5Mn0.5O4−δ investigated in situ by neutron diffraction and X-ray absorption spectroscopy under reducing and oxidizing gas flows

Abstract

The structural and redox stability of the n = 1 Ruddlesden-Popper (RP) oxide Pr0.5Sr1.5Cr0.5Mn0.5O4−δ, synthesized by the citrate-gel method, has been investigated over the temperature range 25–700 °C under reducing (5% H2 flow) and oxidizing (O2 or air flow) conditions by means of in situ neutron powder diffraction (NPD) and X-ray absorption near-edge structure spectroscopy (XANES). Sequential Rietveld refinement of the NPD patterns collected under hydrogen revealed de-intercalation of oxide ions from the equatorial anion positions with retention of I4/mmm symmetry. The reduction from Pr0.5Sr1.5Cr0.5Mn0.5O4.00(2) to Pr0.5Sr1.5Cr0.5Mn0.5O3.81(2) is accompanied by an expansion of both the a and c lattice parameters. When the reduced sample is heated in air, oxygen refills the equatorial sites and the unit cell contracts; the interlayer interstitial site remains unoccupied. XANES showed the oxidation states in the as-prepared composition to be Pr3+, Cr3+ and Mn4+. When the material is heated under dilute hydrogen, the oxidation states Pr3+ and Cr3+ are retained whereas Mn4+ is reduced to Mn3+. These observations constitute the first direct evidence that the d-block element, and not praseodymium, is responsible for the electrocatalytic activity of Pr-containing RP oxides. When the reduced material is heated under oxygen, Mn3+ is reoxidised to Mn4+ and a low concentration of tetrahedrally-coordinated Cr(VI) forms, suggesting a possible poisoning mechanism in fuel-cell applications.