Transport properties and the phase transition in ${\mathrm{Ti}}_{1-x}{M}_x{\mathrm{Se}}_2 (M=\mathrm{Ta}\mathrm{or}\mathrm{V})$

The transport properties of semimetallic Ti${\mathrm{Se}}_2$ are reexamined, and the limits of a two-band model are discussed. We conclude that the density of electrons and holes is 5 × ${10}^{20}$/${\mathrm{cm}}^3$ to 10 × ${10}^{20}$/${\mathrm{cm}}^3$, in agreement with recent photoemission data. The second-order phase transition at 202 K in Ti${\mathrm{Se}}_2$ is suppressed by cation substitution with Ta or V. In both cases, about 7% substitution completely eliminates the transition. However, while Ta appears to donate carriers in the rigid-band sense, V has a magnetic moment which is concentration and temperature dependent. Furthermore, at low V concentrations the resistivity diverges at low temperatures, in contrast to the metallic behavior of the Ta-doped samples. The data suggest that the cation disorder is the major cause of suppression of the transition. Since the disorder is not directly related to the carrier density, we are unable to draw conclusions from these data concerning a possible electron-hole coupling mechanism to drive the transition.