METAL-INSULATOR-TRANSITION IN THE SPINEL CUIR2(S1-XSEX)(4) SYSTEM STUDIED BY NMR

We have investigated the metal-insulator transition (MIT) in Culr(2)(S 1-xSex)(4) by Cu-NMR. The decreases of the nuclear spin-lattice relaxa tion rate, 1/T-1, and of the core-polarization contribution to the Kni ght shift show clearly an evidence of the MIT in the range of x<0.8. T he transition temperature, T-Ml, changes linearly with x(3), indicatin g the predominant atomic size effect on the MIT. The non-magnetic grou nd state below the MIT suggests the nonmagnetic Cu1+ state and implies a possible spin singlet formation for Ir atoms.