Electronic properties and superlattice formation in the semimetal<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">TiSe</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>

Abstract

Neutron-diffraction studies of ${\mathrm{TiSe}}_{2}$ show that a second-order structural phase transition occurs at ${T}_{0}=202$ K involving transverse atomic displacements with wave vector $\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}=(\frac{1}{2},0,\frac{1}{2})$. 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.