Metal-insulator transition at 230 K in a new thiospinel CuIr2S4

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Abstract
A new thiospinel CuIr2S4 has been synthesized. We have discovered a metal-insulator transition at 230 K. The physical properties of this compound have been investigated with special emphasis on heat capacity measurements in the range 13·8 K < T < 300K using an adiabatic calorimeter. The metal-insulator transition is accompanied by a structural phase transition from tetragonal symmetry (at the lower temperature) to cubic symmetry (at the higher temperature). The phase transition has been observed to be first order. A sharp heat capacity anomaly is exhibited 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 the phase transition was detected clearly in the heat capacity measurements. The metallic phase of CuIr2S4 shows Pauli paramagnetism which indicates a rather high density of states at the Fermi level, D(εF) = 0·67 states/eV atom. The insulating phase of CuIr2S4 reveals semiconductive behaviour with an activation energy of 4·7 × 10−2eV in the temperature range 140–200 K. In both the metallic and insulating phases, there is no localized magnetic moment. A possible mechanism for the metal-insulator transition in CuIr2S4 is discussed on the basis of the band Jahn-Teller effect.

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