Metal-insulator transition in the spinel-type Cu(Ir1−xVx)2S4

The spinel CuIr2S4 exhibits a temperature-induced metal-insulator (M-I) transition at around TM−I=226 K with a structural transformation, at which a simultaneous bond dimerization with spin-singlet state and charge-ordering transition takes place. Conversely, CuV2S4 exhibits two-step anomalies at about 92 and 56 K, reflecting the CDW state. High-purity spinel-type Cu(Ir1−xVx)2S4 specimens have been synthesized. The lattice constant does not obey Vegard's law, but looks like a parabolic curve. The d electrons on the octahedral B sites play a dominant role in determining the physical properties. A low substitution (x≈0.04) of V for Ir leads to disappearance of the step-like sharp M-I transition, and vice versa, a low substitution of Ir for V destroys the CDW transition. The local structural and orbital degrees of freedom in Cu(Ir1−xVx)2S4 couple to the spin system and the electronic state. The M-I transition and the CDW state are incompatible with each other in Cu(Ir1−xVx)2S4. In the metallic phase, a novel relationship, namely “a mirror image effect” has been manifestly found between the residual resistivity and the electronic energy density-of-states D(εF) at the Fermi level. The lower the resistivity, the higher the value of D(εF). Furthermore, a composition-induced gradual M-I transition is found over the range of 0.00⩽x⩽1.00, where the most insulating behavior is displayed around x≈0.2-0.3.