High-temperature redox chemistry of La1.5+xSr0.5-xCo0.5Ni0.5O4+delta (x = 0.0, 0.2) studied in situ by neutron diffraction

Neutron powder diffraction has been used to investigate the redox behaviour of two n = 1 Ruddlesden-Popper (RP) oxides, La1.5+xSr0.5-xCo0.5Ni0.5O4+delta (x = 0.0, 0.2) in situ and in real time during cycling through the temperature range 20 <T/degrees C <600 under flowing 5% H-2. Both compositions retained I4/mmm symmetry throughout the heating and cooling cycle. Rietveld refinement of data collected at room temperature showed the presence of interstitial oxygen in the original samples (delta approximate to 0.06(1) and 0.12(1) for x 0.0 and 0.2, respectively). When heated in flowing H-2, both compositions were reduced over the temperature range 300 <T/degrees C <600 (delta approximate to -0.22(2) and -0.06(2) for x 0.0 and 0.2, respectively). In the case of the x = 0.2 material, reduction clearly occurred first at the interstitial site (O-int), then at the equatorial site (O-eq). The changes in composition were accompanied by changes in unit-cell parameters and bond lengths. Rietveld refinements revealed residual partial occupation of the O-int site in the reduced x = 0.2 sample. This is the first structural evidence of the coexistence of both anion vacancies and interstitial anions in a hypostoichiometric n = 1 RP oxide. The temperature dependence of the atomic displacement parameters is consistent with the presence of local rotations of the anion-deficient coordination polyhedra. On heating the reduced x = 0.2 material in an O-2 flow, the refilling of both the O-eq and O-int sites began at 200 degrees C and the stoichiometry of the original as-prepared sample was recovered by 250 degrees C.