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 Nd_2−xSr_xNiO_4+δ and Pr_2−xSr_xNiO_4+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 Pr₄Ni₃O_10−x, or RE_xO_y intergrowth phases, rendering the title compounds unstable even under ambient conditions. Related to potential applications as membranes in SOFC, we importantly found Nd_2−xSr_xNiO_4+δ and Pr_2−xSr_xNiO_4+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/Pr₄Ni₃O_10−x or RE_xO_y intergrowth phases are discussed as a drawback limiting high temperature stability for Pr₂NiO_4+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 Pr₂NiO_4.25 and Nd₂NiO_4.25 with a 2D modulation vector related to oxygen ordering.