Electronic and vibronic structure of TiS2 and TiSe2

A comparison of the diverse properties of 1T-TiSe2 and 1T-TiS2 shows interesting similarities and differences. Electron diffraction on both materials at room temperature, reveals a hexagonal pattern except for diffuse streaking suggestive of short range 2a0 × 2a0 order. A commensurate 2a0 × 2a0 × 2c0 superlattice forms in the selenide below Tc ⋍ 200K. So far, a locked-in superlattice of this type has not been observed in the sulphide. Good crystals of both materials have similar conductivity at room temperature but differ greatly at a low temperature. In particular, the phase transition in the selenide has profound effects on DC conductivity and also on infrared reflectivity, which has been measured as a function of temperature in both stoichiometric and nonstoichiometric crystals over a wide frequency range 40 to 4000 cm-1. Both infrared and Raman scattering data on TiSe2 below Tc can be well correlated with folding of zone boundary frequencies as measured by inelastic neutron scattering. It does appear that quasi soft-mode behavior of certain vibrational branches occurs in connection with the periodic lattice distortion of the selenide. On the other hand, changes in the electronic structure must also be important. Recent angle resolved photoemission measurements on both compounds confirm that TiSe2 is a semimetal with small band overlap, whereas TiS2 is a defect semiconductor. The experimental results are in remarkable agreement with recent self-consistent energy band calculations, which detail the electron-hole negative energy gap behavior of TiSe2 and the small positive energy gap in TiS2.