Thermoelectric performance of indium-rich thiospinels In1 y.yIn2S4 xTex (y ∼ 0.16, 0.22 and x ∼ 0.1, 0.2)

Powder x-ray diffraction and Raman spectroscopy studies showed In-rich thiospinels with the chemical formula In1 y ❑yIn2S4 xTex (y approximate to 0.16, 0.22 and x approximate to 0.1, 0.2) to be composed of the cubic a-modification [space group (SG) Fd3m] with a minor admixture of tetragonal fl -phase (SG I41/amd). Tellurium incorporation in the crystal structures is confirmed by Rietveld refinements, energy dispersive x-ray-and Raman spectroscopies. All samples are found to be indirect band gap semiconductors with an Eoptg approximate to 2 eV, which decreases with increasing Tecontent. The thiospinels with y approximate to 0.16, 0.22 and x approximate to 0.1 revealed by two orders of magnitude enhanced charge carrier concentration (i.e., n approximate to 1018 cm  3) and approximate to 6-9 times larger charge carrier mobilities (mu) than corresponding binary specimens. A kink at 200 K in p(T) is found to originate from a stepper increase of n. The Seebeck coefficient is negative for all samples indicating electrons to be the dominant charge carriers. The total thermal conductivity of the compounds is almost completely phonon (Kph) mediated. The Debye-Callaway fits of Kph reveal an enhanced point-defect scattering in the In2.78S4 xTex series compared to In2.84S4 xTex, in accordance with the higher vacancy concentration (i.e., smallery). The Balkanski-Klemens analysis of the temperature dependent Raman shifts indicated domination of three-phonon scattering mechanism agreeing with the lowered Kph values. The appearance of low-energy optical modes confirms the In1-atoms, randomly occupying [In1S4]-tetrahedra, to reveal a 'rattling' motion and thus, to be a probable source of enhanced anharmonic effects limiting Kph. Te-doping of In-rich thiospinels (y approximate to 0.16, 0.22) results in the improvement of the electrical transport characteristics and simultaneously in an increase of their thermal conductivities due to smaller vacancy concentration. The possible reduction of Kph and the improvement of the TE performance are discussed.