The crystal structure of spinel compound CuIr
2S
4 was investigated by combination of high-resolution x-ray, electron and neutron diffraction measurements. CuIr
2S
4 is well known to show metal-insulator (MI) transition accompanied by structural transition at
TMI-230 K. The electrical resitivity abruptly increases at
TMI, on cooling and the localized magnetic moment vanishes simultaneously. In the present study, the crystal structure below
TMI is successfully determined. The results show that CuIr
2S
4 undergoes a simultaneous charge-ordering and spin-dimerization transition, which can explain the physical properties of CuIr
2S
4. The charge-ordering pattern consists of isomorphic Ir
83++S
24 and Ir
84++S
24. This arrangement is a new description of the spinel structure but much more complex than the known charge-ordering patterns such as stripes.
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