Synthesis,Structure,and Physical Properties of Cobalt Perovskites:Sr-3CoSb_2O_9 and Sr_2CoSbO_(6-#delta#)

摘要

The perovskites Sr3CoSb2O,, Sr2CoSbO6, and Sr2CoSbO563 have been synthesized in polycrystalline form by solid state reactions from acetate precursors as obtained by freeze-drying. The crystal structure of insulating Sr3CoSb2O9 has been refined from X-ray data by the Rietveld method and was, found to be orthorhombic (Immm) with a = 5.6411(1) A, b = 5.6610(1) A, c = 7.9829(2) A. From the molar susceptibility curves an effec-tive magnetic moment Peff = 5.39 p~ and a spin-glass transition at ca. 6 K are deduced. The semiconducting perovskite Sr2CoSbO6 was synthesized by applying an elevated 02 pressure (3 kbar), and Co exclusively in the oxidation state +3 was attained. The Rietveld refinement of this compound has revealed a rhombohedral cell (R~m) with a = 5.5992(2) A and c = 13.6609(2) A and partial order (77%) of the B cations. An effective magnetic moment per formula unit of ~ = 3.83 Pu and a spin-glass transition at ca. 47 K were observed. A semiconduct-ing and oxygen-deficient perovskite, Sr2CoSbO56,, was obtained by annealing Sr2CoSbO6 at 9500C in Ar. The Rietveld refinement proved it to be a double-cubic perovskite (Fm~m) with a = 7.%58(1) A, and a higher order of Co and Sb at the B sites (84%) was found, due to the formation of Co~(+3).