Ti_<1-x>Hf_xSe_2系の半導体的性格と相転移

The electrlcal resistivity ρ and Hall coefficient R_H have been investigated on pressed powder samples of Ti_<1-x>Hf_xSe_2 and sublimation grown crystals of TiSe_2 from 22 K to 300 K. With decreasing temperature the resistivity exhibits a peak below tbe onset temperature of the superlattice in the case of small x. Then at lower temperatures a semiconductinglike resistivity is observed, suggesting that the initial band gap is positive in the mixed system Ti_<1-x>Hf_xSe_2. While the resistivity ρ at 22 K　is also found to be larger than ρ at 300 K for both pressed samples and sublimation grown crystals of TiSe_2, this increase appears to be due to internal strain possibly present in the samples. The results indicate that the mechanism for driving the phase transition is mainly relevant to an excitonic electronhole instability.