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

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 R3¯¯¯. Electron beam irradiation led to a phase transformation from space group R3¯¯¯ to P3¯¯¯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.