Thermoelectric properties of mixed layered compounds TiS2-xSex (0≤×≤2)

Seebeck coefficients (S) were measured in TiS_2-xSe_x for 0≤×≤2 in the temperature range 77-285 K. For TiS₂ (x=0), S was negative and its absolute value increased linearly with temperature, attaining to 170∼250μ V/K, depending on individual samples, at room temperature. With increasing x, the absolute value of temperature coefficient (TC) of S decreased in the temperature region T ≥ 170K. At x ≈ 1.5, TC was reduced to zero and changed its sign for larger x. This behavior is explained by the fact that the valence band of this system consists of S 3p and/or Se 4p bands and shifts upward with increasing x. For TiSe₂ (x=2), S became positive for T ≥ 270K due to the increase in the effect of valence band with increasing temperature. The charge density wave (CDW) transition at 200K in TiSe₂ opens the CDW gap and pushes down the lower edge of CDW gap with decreasing temperature. Below 150K, S is controlled by the upper CDW band consisting of Ti 3d band. Thermoelectric behavior of this system can be totally explained by the above band picture. The present experiments suggest a possibility of raising S in TiS₂ by a substitution for Ti and by an itercalation to modulate the 3d band and to control the Fermi level.