The pressure dependences of TiS₂and TiSe₂band structures

The band structures of the layer compounds 1T-TiS2 and 1T-TiSe2 have been calculated within the local-density approximation using the self-consistent LAPW method. Both compounds are found to be semimetals with Gamma -L overlaps of 0.24 and 0.55 eV, respectively. The positioning and occupation of the bands near the Fermi level are not consistent with the carrier densities determined from Hall measurements; however, agreement is obtained when the overlap is decreased by about 0.25 eV. Calculations performed with contracted lattice constants reproduce qualitatively the same trends in Hall coefficients as observed experimentally. However, quantitative agreement cannot be obtained without an overlap decrease of about 0.25 eV-indicating that TiSe2 is a semimetal and TiS2 a semiconductor.