Single Crystal Growth of Sulphospinel CuIr_2S_4 which exhibits the metal-insulator transition

点击翻译按钮获取中文摘要

The normal sulphospinel CuIr_2S_4 exhibits a temperature-induced metal-insulator (M-I) transition around 226 K with structural transformation, showing hysteresis on heating and cooling With decreasing temperature, a symmetry change occurs from cubic to tetragonal symmetry and a change from the Pauli paramagnetism in metallic behavior to diamagnetism due to the atomic core orbital. A gap in the electronic density of states opens below the M-I transition temperature. It has been verified that d-electrons of Ir atom on the octahedral B-sites have a significant role for the M-I transition.Single crystals of a sulphospinel CuIr_2S_4 have been successfully grown from bismuth solution by a slow cooling method for the first time. The grown crystals have a maximum edge of about 1 mm in size and a mirror-like shining surface. Optimum growth conditions are fairly strict. The specific weight of starting materials for the crystal growth is found to be 0.30 g of CuIr_2S_4 and 10.0 g of Bi in order to obtain good quality crystals. The starting and finishing temperatures for the slow cooling step in the temperature control are 1000 and 500 ℃. The pertinent cooling rate is 〜2 ℃/h. Since the volume of bismuth itself expands in the transition from liquid phase to solid phase, a simple method of separation of the grown crystals from the liquid solution is proposed for avoiding the mechanical damages to the grown crystals. The single crystals have the normal-spinel structure of the lattice constant a =9.849 Å at room temperature. A step-like anomaly in the susceptibility of the single crystals, corresponding to the metal-insulator transition in the resistivity, occurs much sharply than in the powder specimen.