The effect of Mg substitution for Ti on transport and thermoelectric properties of TiS_2

摘要

Magnesium substituted compounds Mg_xTi_(1-x)S_2 (0≤x≤0.15) were prepared by solid-state reaction, and their transport and thermoelectric properties were investigated from 5 to 310 K. The results indicate that at low temperatures (T＜～175 K), a transition from metallike to semiconductorlike behavior occurred after the substitution of Mg for Ti, which implies that intrinsically TiS_2 is a semiconductor and this transition can be ascribed to de-degeneration through the reduction in electron concentration due to Mg~(2+) substitution for Ti~(4+). Furthermore, it was found that dc conductivity σ for Mg_xTi_(1-x)S_2 (x＞0) obeyed Mott's two-dimensional (2D) variable range hopping law In σ ∝ T~(-1/3) at T＜～50K, indicating that TiS_2 possess 2D transport characteristics. The appearance of Mott's 2D law could originate from a potential disorder introduced by Mg substitution for Ti in S-Ti-S slabs. Meanwhile, the significant enhancement of absolute thermopower of Mg_xTi_(1-x)S_2 (x＞0) in the whole temperature range investigated could also be attributed to the reduction of electronic concentration after doping. The thermoelectric figure of merit ZT of heavily substituted compounds (x=0.10 and 0.15) was smaller than that of TiS_2, owing to the large increase of both their electrical resistivity and (lattice) thermal conductivity presumably caused by the reduced electron concentration and increased acoustic velocity, respectively. Nevertheless, ZT of the lightly substituted compound Mg_(0.04)Ti_(0.96)S_2 enhanced substantially due to the remarkable increase in its thermopower, and specifically it is ～ 1.6 times as great as that of TiS_2 at 300 K, indicating that doping (substitution) is an effective approach to enhance thermoelectric performance of TiS_2.