METAL-INSULATOR-TRANSITION AT 230 K IN A NEW THIOSPINEL CUIR2S4

A new thiospinel CuIr2S4 has been synthesized. We have discovered a me tal-insulator transition at 230 K. The physical properties of this com pound have been investigated with special emphasis on heat capacity me asurements in the range 13.8 K < T < 300 K using an adiabatic calorime ter. The metal-insulator transition is accompanied by a structural pha se transition from tetragonal symmetry (at the lower temperature) to c ubic symmetry (at the higher temperature). The phase transition has be en observed to be first order. A sharp heat capacity anomaly is exhibi ted at 230.8 K. The enthalpy and entropy of the transition are 3.50 kJ mol(-1) and 15.6 J K-1 mol(-1) respectively. A hysteresis effect at t he phase transition was detected clearly in the heat capacity measurem ents. The metallic phase of CuIr2S4 shows Pauli paramagnetism which in dicates a rather high density of states at the Fermi level, D(epsilon( F)) = 0.67 states/eV atom. The insulating phase of CuIr2S4 reveals sem iconductive behaviour with an activation energy of 4.7 X 10(-2) eV in the temperature range 140-200 K. In both the metallic and insulating p hases, there is no localized magnetic moment. A possible mechanism for the metal-insulator transition in CuIr2S4 is discussed on the basis o f the band Jahn-Teller effect.