Growth of high quality single crystals of strontium doped (Nd,Pr)-nickelates, Nd2-xSrxNiO4+δ and Pr2-xSrxNiO4+δ

Abstract

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.