Stoichiometry dependence of the transport properties of TiS₂

Transport and Raman data are presented for samples of Ti _1+xS₂ with carrier densities, determined from the Hall coefficient, ranging from approximately 10²⁰ cm^-3 to approximately 3*10²¹ cm^-3. No dependence on carrier concentration, n, is observed for the frequency of the A_1g Raman-active phonon, whereas a strong variation is seen in the non-linear temperature dependence of resistivity, rho (T), between 4.2K and 290K. Fitting the resistivity to a power law, rho (T)- rho (o) varies as T^p, in the range 110K to 290K, the authors find p=2.3 for n approximately 10²⁰ cm^-3 and p=1.6 for n approximately *10²¹ cm ^-3. For samples with carrier concentrations above approximately 7*10²⁰ cm^-3 a stronger temperature dependence is observed below 100K. The temperature-dependent part of the resistivity is proportional to n^-0.74 at room temperature and n^-0.60 at 150K. It is argued that the scattering mechanism in TiS₂ is with both optical and acoustic phonons. Below the Debye temperature the form and variation with carrier concentration of the temperature dependence may be described by a model involving both intra- and inter-valley scattering by acoustic phonons.