Low-Temperature Iodine Heat Treatment of Quasi-One-Dimensional KFeS ₂ Crystals

Low-temperature iodine heat treatment of quasi-one-dimensional KFeS ₂ crystals was carried out. The resistivity along the c-axis is seen to decrease rapidly with increasing iodine-heat-treatment temperature and the nonmetallic temperature dependence was found to become weaker. The fluctuation-induced tunneling model was used to explain the reduction of the potential barrier at grain boundaries caused by the iodine heat treatment. In particular, samples subjected to iodine heat treatment at 100°C for 2 days exhibited a low room-temperature resistivity of approximately 10^-1 Ωcm and a corresponding positive temperature gradient, which are evidence of the first observation of metallic behavior. Furthermore, the room-temperature Seebeck coefficient of these iodine-heat-treated samples was small, a value of +5.8 μV/K, and exhibited an almost linear temperature dependence that showed the samples to be metallic. Finally, although KFeS₂ normally exhibits metallic conduction, the presence of grain boundaries formed during crystallization from the molten state is thought to have led to the nonmetallic conductivity observed in our experiments.