Ionic Ordering in Thiospinel CuIr₂S₄

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Crystal structure of spinel compound CuIr₂S₄ was examined by powder X-ray diffraction for the insulating phase below the metal-insulator transition at TMI = 230 K. The superstructure spots are reproduced by considering the displacement of Ir atoms. A model for the ionic ordering of Ir⁴⁺ and Ir³⁺ with the same number is proposed for the insulating phase on the basis of the structural analysis. The model suggests that the structural change at TMI is driven by the formation Ir⁴⁺ dimers. In addition, we found that the superstructure spots becomes significantly weak below 60 K, without any significant effects on electric and magnetic properties. Possible mechanism for the transition is discussed.