Influence of covalence on the metal-insulator temperature in the CuIr2(S1-xSex)4 spinels

Influence of the covalence on the metal-insulator (M-I) temperature in the CuIr2(S1-xSex)(4) spinel series is considered. For this purpose a vacancy model and the effective number of valence electrons per molecule are used. Calculations of the vacancy model parameters, i.e., the ion packing coefficients, the filling coefficient of the unit cell, the difference of the Pauling electronegativities and the vacancy parameter suggest that the covalence increases with increasing selenium concentration x, leading to a decrease of the metal-insulator temperature. It was observed that the metal-insulator temperature is related to the effective number of valence electrons per molecule, suggesting that with increasing covalence, less and less valence electrons take part in the metal-insulator transition. These effects are explained within the framework of the mixed valence of the iridium ions including structural defects.