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The crystal and magnetic structures of an ordered double perovskite, Ca 2 FeReO 6 , were studied by high-resolution neutron powder diffraction as a function of the temperature from 7 K to 550 K. All of the diffraction data were precisely refined by the Rietveld method, and we confirmed a structural phase transition at around 140 K where the metal-insulator transition occurs from ferrimagnetic metal (FM) to ferrimagnetic insulator (FI) phases. At this temperature, there exists a change in the distortion direction of [ReO 6 ] octahedra together with a spin reorientation, which strongly supports the occurrence of orbital ordering of the t 2g electrons. FM and FI phases coexist in a narrow temperature range at around 140 K, which is typically seen in the first-order phase transition. A phase separation was not detected in our well-characterized sample.

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