Thermoelectric properties of mixed layered compounds TiS{sub}(2-x)Se{sub}x(0≤x≤2)

摘要

Seebeck coefficients (S) were measured in TiS{sub}(2-x)Se{sub}x for 0≤x≤2 in the temperature range 77-285K. For TiS{sub}2 (x = 0), S was negative and its absolute value increased linearly with temperature, attaining to 170～250μV/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 TiSe{sub}2 (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 TiS{sub}2 by a substitution for Ti and by an itercalation to modulate the 3d band and to control the Fermi level.