HALL-COEFFICIENT AND REFLECTIVITY EVIDENCE THAT TIS2 IS A SEMICONDUCTOR

A series of measurements of the Hall coefficient, infrared reflectivity, thermoelectric power and electrical resistivity of Ti1+xS2 single crystals with various degrees of stoichiometry is described, where, for the first time, each measurement was made on the same crystal (or crystals from the same batch). None of these measurements taken alone can distinguish between the semimetallic or semiconducting models of TiS2. However, by making all four measurements on each sample, it has been possible to establish correlations between the results for different samples. It was found that the product of the Hall coefficient and the square of the plasma frequency is the same for all samples, a result that is consistent with a semiconductor model, but is inconsistent with a semimetal. Nevertheless the most stoichiometric samples remain metallic with electron concentrations of 2*1020 cm-3. It was also found that the resistivity data cannot be explained by carrier-carrier or optical phonon scattering. Therefore, both the source of the residual conduction electrons and the scattering mechanism in TiS 2 remain unknown.