Semiconductor to semimetal transition in TiS₂ at 40 kbar

Evidence for p-d band crossing at a pressure of 40 kbar is presented for the layered compound TiS₂, based on the variation with pressure of the Hall coefficient, R_H, thermoelectric power, S, and resistivity, rho . Below 40 kbar the transport properties may be understood within the framework of an extrinsic semiconductor model with strong electron-phonon scattering. Under pressure the band overlap is estimated to change at the rate of 4.5+or-1.5 meV kbar^-1; this implies a band gap of 0.18+or-0.06 eV for TiS₂ at ambient pressure. Analysis of the quantity -R _H/ rho ²e as a function of pressure, and solution of the transport equations at 90 kbar are both used to show that in the semimetallic state of TiS₂ the d-band electron mobility exceeds the p-band hole mobility. In contrast, the hole mobility is the greater at all pressures up to 90 kbar in TiSe₂.