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Author(s): Y. Tsujimoto [1]; C. Tassel [1, 2]; N. Hayashi [3]; T. Watanabe [1]; H. Kageyama (corresponding author) [1]; K. Yoshimura [1]; M. Takano [4, 5]; M. Ceretti [2]; C. [...]

We investigated the structure of Nd2NiO4+δ and Pr2NiO4+δ by single crystal neutron diffraction studies. While the real structure of both compounds is incommensurate, the scattering density of the respective average structures was explored using the Maximum Entropy Method. Unusually high displacement factors were found for the equatorial and apical oxygen atoms showing respectively large displacement amplitudes towards [001] and [110] with respect to the F-symmetry cell. The shifts of the apical oxygen atoms reach up to 1 Å from their average position, corresponding to a 25° tilt of the NiO6 octahedra. At 400 °C, i.e. slightly above the orthorhombic-tetragonal phase transition, the anharmonic apical oxygen displacements towards [110] in the commensurate tetragonal parent structure are strongly enhanced, showing a double-well potential and pointing towards the interstitial vacancy sites, creating a quasi continuous shallow energy diffusion pathway between apical and interstitial oxygen sites. These large displacement amplitudes are considered to be – at least partially – of dynamical origin, which is consistent with a phonon assisted diffusion mechanism, already activated at very moderate temperatures. [ABSTRACT FROM AUTHOR]

Large size and high quality single crystals of Nd2−xSrxNiO4+δ and Pr2−xSrxNiO4+δ, with selected composition (x = 0.0, 0.1 and 0.5), were grown by a floating-zone technique using an image furnace. We found that even small deviations from the ideal cation stoichiometry led to either NiO segregation or formation of Ni-poor Pr4Ni3O10−x, or RExOy intergrowth phases, rendering the title compounds unstable even under ambient conditions. Related to potential applications as membranes in SOFC, we importantly found Nd2−xSrxNiO4+δ and Pr2−xSrxNiO4+δ powders, obtained from ground stoichiometric single crystals, to be stable in air at high temperature (1000 °C), contrary to the partial decomposition observed when synthesized as polycrystalline powders by classical solid state synthesis. The presence of NiO/Pr4Ni3O10−x or RExOy intergrowth phases are discussed as a drawback limiting high temperature stability for Pr2NiO4+δ. Grown single crystals were characterised by neutron and X-ray diffraction as well as by electron microscopy (SEM/EDS), revealing their excellent quality in terms of composition, homogeneity and crystallinity. For each crystal, the oxygen content was determined by thermogravimetric analysis in a reductive atmosphere. Transmission electron microscopy highlighted a complex incommensurate structure for Pr2NiO4.25 and Nd2NiO4.25 with a 2D modulation vector related to oxygen ordering. [ABSTRACT FROM AUTHOR]

Large size and high quality single crystals of Nd2−xSrxNiO4+δ and Pr2−xSrxNiO4+δ, with selected composition (x = 0.0, 0.1 and 0.5), were grown by a floating-zone technique using an image furnace. We found that even small deviations from the ideal cation stoichiometry led to either NiO segregation or formation of Ni-poor Pr4Ni3O10−x, or RExOy intergrowth phases, rendering the title compounds unstable even under ambient conditions. Related to potential applications as membranes in SOFC, we importantly found Nd2−xSrxNiO4+δ and Pr2−xSrxNiO4+δ powders, obtained from ground stoichiometric single crystals, to be stable in air at high temperature (1000 °C), contrary to the partial decomposition observed when synthesized as polycrystalline powders by classical solid state synthesis. The presence of NiO/Pr4Ni3O10−x or RExOy intergrowth phases are discussed as a drawback limiting high temperature stability for Pr2NiO4+δ. Grown single crystals were characterised by neutron and X-ray diffraction as well as by electron microscopy (SEM/EDS), revealing their excellent quality in terms of composition, homogeneity and crystallinity. For each crystal, the oxygen content was determined by thermogravimetric analysis in a reductive atmosphere. Transmission electron microscopy highlighted a complex incommensurate structure for Pr2NiO4.25 and Nd2NiO4.25 with a 2D modulation vector related to oxygen ordering. [ABSTRACT FROM AUTHOR]