In situneutron diffraction study of the high-temperature redox chemistry of Ln3−xSr1+xCrNiO8−δ (Ln = La, Nd) under hydrogen

Abstract

The chemical reduction of the K₂NiF₄-type oxides, Ln₂Sr₂CrNiO_8−δ (Ln = La, Nd) and Nd_2.25Sr_1.75CrNiO_8−δ, has been investigated in situ under a dynamic hydrogen atmosphere at high temperature using neutron powder diffraction. The high count-rate and high resolution of the D20 diffractometer at ILL, Grenoble allowed real-time data collection and structure refinement by full-pattern Rietveld analysis with a temperature resolution of 1 °C. Excellent agreement was obtained with the results of thermogravimetric analysis of these materials, which are potential fuel-cell electrodes. The neutron study revealed that oxygen is lost only from the equatorial anion site; the reduction of La₂Sr₂CrNiO_8−δ yields a pure Ni(II) phase, La₂Sr₂CrNiO_7.5en route to a mixed Ni(II,I) oxide, La₂Sr₂CrNiO_7.40, whereas hydrogen reduction of Nd₂Sr₂CrNiO_8−δ and Nd_2.25Sr_1.75CrNiO_8−δ proceeds continuously from Ni(III) to an average oxidation state of 1.80 for the nickel ion. The data collected throughout a subsequent heating/cooling cycle in air indicated that the reduced phases intercalate oxygen reversibly into the equatorial vacancies of the K₂NiF₄-type structure. The retention of I4/mmm symmetry, along with the absence of the formation of any impurities throughout the heating/cooling cycles under reducing and oxidizing atmospheres, demonstrates both the reversibility and the strongly topotactic character of the oxygen deintercalation/intercalation chemical redox process and establishes the excellent structural stability of these layered mixed-metal oxides over a wide range of oxygen partial pressures.