Electronic properties and superlattice formation in the semimetal ${\mathrm{TiSe}}_2$

Neutron-diffraction studies of ${\mathrm{TiSe}}_2$ show that a second-order structural phase transition occurs at $T_0=202\mathrm{}$ K involving transverse atomic displacements with wave vector $\overrightarrow{\mathrm{q}}=(\frac{1}{2},0,\frac{1}{2})$. The electronic transport properties of the most nearly stoichiometric crystals show that ${\mathrm{TiSe}}_2$ is a semimetal with $n_e=n_h={10}^{20}$/${\mathrm{cm}}^3$. Small impurity concentrations or deviations from stoichiometry reduce $n_h$, increase $n_e$, and suppress the phase transition. This reduction together with the observed displacement pattern lead us to speculate that the transition is driven by an electron-hole coupling.