Thermoelectric properties of mixed layered compounds TiS2-xSex (0≰x≰2)

Seebeck coefficients (S) were measured in TiS2-xSex for 0less than or equal toxless than or equal to2 in the temperature range 77-285K. For TiS2 (x=0), S was negative and its absolute value increased linearly with temperature, attaining to 170similar to250muV/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 Tgreater than or equal to170K. At xapproximate to1.5,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 Tgreater than or equal to270K 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.