Correlation between the Sign of Hall Coefficient and Lattice Instabiliy in Substituted TiSe_2

The temperature dependence of the electrical resistivity and the Hall coefficient at 290 K for single crystals of Ti_<1-x>f_xSe_2 are presented. The resistive anomaly due to lattice instability appears in the range of 0 ≤ x ≤ 0.35 where the Hall coefficient is found to be positive. The sign of the Hall coefficient is changed to minus for x ≥ 0.4. The results show that holes as carriers play a fundamental role in the lattice instability in TiSe_2.