Does a Low Amount of Substituents Improve the Thermoelectric Properties of Cr2−xMxS3 (M = Ti, V, Sn)?

Abstract

The effects of low-level partial cation substitution in Cr2−xMxS3 with M = Ti, V or Sn and x = 0.05 and 0.1 have been investigated regarding the long- and short-range crystal structures and thermoelectric properties. All substituted compounds crystallized in the equilibrium phase of Cr2S3, adopting the space group R$${\overline{\text{3}}}$$ 3 ¯ . Electron beam irradiation led to a phase transformation from space group R$${\overline{\text{3}}}$$ 3 ¯ to P$${\overline{\text{3}}}$$ 3 ¯ 1c with a subsequent appearance of diffuse scattering, indicating short-range ordering of cations in the partially occupied cation layers. Substitution of Cr by V led to a reduction in electrical conductivity and subsequently to a lower thermoelectric performance in comparison to the pristine material. In contrast, substitution with Ti yielded an improvement of the performance due to a higher electrical conductivity and a reasonably high Seebeck coefficient. Both Sn-substituted compounds contained only traces of Sn. Surprisingly, a significant improvement of the electrical conductivities could be observed in comparison to the pristine material as well as the other Cr2−xMxS3 materials.

About the Impact of Defect Phases on the Thermoelectric Properties of Cr3S4–xSex

“…As expected for a metallic conductor, the electrical conductivity decreases from around 1300 Scm −1 at RT to 960 S cm −1 at 773 K. In contrast, the Seebeck coefficient slightly increases up to −51 μV K −1 at 773 K. Unusually, the thermal conductivities increase with temperature from 2.3 at RT to 3–3.3 Wm −1 K −1 at 723 K. A similar behavior has already been observed for Cr 2 S 3 . [ 30 ] Overall, the values for the thermoelectric figure of merit ( ZT ) are at a very low level with 0.01 at RT and a maximum of 0.06 at 773 K. Although in the non‐sintered sample the defect phase could be identified at 623 K by in situ XRD, no differences in the values between the measurements can be observed. Thus, the sintering appears to have a positive effect on the structural stability and prevents the formation of the defect phase if it was not yet present at RT.…”

“…The tripled parameter does not correspond to the R equilibrium phase, but occurs in the second polytype of Cr 2 S 3 with space group P 1 c. This polytype has been observed to occur after an electron beam–induced phase transformation of Cr 2 S 3 , which has been reported in previous studies. [ 25,30 ]…”

“…This polytype has been observed to occur after an electron beaminduced phase transformation of Cr 2 S 3 , which has been reported in previous studies. [25,30] A general TEM investigation has also been performed on a sample with the nominal composition Cr 3 S 2 Se 2 , to determine the general composition and structure after substitution. No significant deviations in stoichiometry could be identified via EDX, with d-values determined via SAED generally being measured right between those of Cr 3 S 4 and Cr 3 Se 4 .…”