Local Ir dedimerization followed by ligand-hole hopping in X-ray irradiated thiospinel

The extended X-ray-absorption fine structure of a spinel CuIr₂(S0.95Se_0.05)₄ powder sample, which has the charge-ordered temperature T_CO=190 K, was determined at the Ir L₃ edge for a wide temperature range. All the local structure parameters on the Ir–Ir bonds signified the second structural change at T^*~70 K. Remarkably, the number of Ir dimers, N_D(T), decreased by ~10% below T^* [N_D(0)/N_D(T^*)~0.9], which indicates partial dedimerization by X-ray irradiation. We also reanalyzed the resistance data of CuIr₂S₄ [Furubayashi et al., Solid State Comm. 126, 617 (2003)] with the variable-range hopping model by focusing on the density of states at E_F, g. The X-ray-induced decrease in resistance was quantitatively explained as a result of the increase in g, which was attributed to the release of holes from the 10%-dedimerized Ir ions. We suppose that the released holes hop over the ligand chalcogen site and discuss the origin of X-ray-induced structural modulations [Kiryukhin et al., Phys. Rev. Lett. 97, 225503 (2006)] in terms of segregation of broken dimers.