Dimerization and charge ordering state in the spinel-type CuIr₂S₄under high pressure

Spinel-type CuIr₂S₄ exhibits charge ordering and dimerization accompanied by structural phase transitions at low temperatures [1]. Recent studies have reported that CuIr₂S₄ exhibits high-temperature superconductivity with a maximum T_c of 18 K under high pressure, along with multiple structural phase transitions [2]. However, the crystal structure under high pressure remains unclear. In this study, X-ray diffraction experiments conducted at 300 K revealed successive structural phase transitions occurring up to 25 GPa. The “low-pressure phase” appearing around 3-16 GPa was found to exhibit the same charge ordering and dimerization pattern observed at ambient temperature and pressure. On the other hand, the “high-pressure phase” that appears at pressures above 12 GPa had an unknown structure. By combining unit cell information obtained from Le Bail analysis with structural optimization calculations, we clarified that the “high-pressure phase” has a different charge order and dimerization pattern from the “low-pressure phase”. Unlike the “low-pressure phase”, the charge order pattern of the “high-pressure phase” satisfies the Anderson condition, suggesting that the increase in Coulomb repulsion due to pressure drove the system toward a charge order structure that is more stable from the perspective of Coulomb interaction.

[1] B. Chen et al., arXiv:2411.04407, [2] T. Ohashi et al., Phys. Rev. B. 111, 224114 (2025).