STRUCTURAL AND MAGNETIC STUDIES OF METAL-INSULATOR-TRANSITION IN THIOSPINEL CUIR2S4

Crystal structure and magnetic susceptibility were investigated for a thiospinel compound CuIr2S4, which exhibits a transition from metallic to insulating on cooling at the transition temperature T-c of 230 K. It was found that the transition is accompanied by a first-order struc tural transition. As the compound is cooled through T-c, the structure transforms from cubic to tetragonal with a volume contraction of 0.7% . The tetragonal phase has an axial ratio of about 1.03. Magnetic susc eptibility, which is paramagnetic in the metallic region, decreases sh arply and becomes diamagnetic at T-c on cooling. The susceptibility is mostly temperature independent in each region above and below T-c. Ma gnetic measurements also revealed that high pressure stabilizes the in sulating phase, in contrast with most of the compounds that exhibit me tal-insulator transitions. Possible mechanisms of the metal-insulator transition are discussed.