Ionic Ordering in Thiospinel CuIr${}_2$S${}_4$

Abstract

Crystal structure of spinel compound CuIr${}_2$S${}_4$ was examined by powder X-ray diffraction for the insulating phase below the metal-insulator transition at $T_{MI}$ = 230 K. The superstructure spots are reproduced by considering the displacement of Ir atoms. A model for the ionic ordering of Ir${}^{4+}$ and Ir${}^{3+}$ 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 $T_{MI}$ is driven by the formation Ir${}^{4+}$ 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.

10 pages, 4 figures, submitted to J. Phys. Soc. Japan