Thermoelectric performance of indium-rich thiospinels [formula omitted] ([formula omitted] ≈ 0.16, 0.22 and x ≈ 0.1, 0.2)

Abstract

Powder x-ray diffraction and Raman spectroscopy studies showed In-rich thiospinels with the chemical formula In1−y□yIn2S4−xTex (y ≈ 0.16, 0.22 and x ≈ 0.1, 0.2) to be composed of the cubic α-modification [space group (SG) Fd3̅m] with a minor admixture of tetragonal β-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 Egopt≈ 2 eV, which decreases with increasing Te-content. The thiospinels with y ≈ 0.16, 0.22 and x ≈ 0.1 revealed by two orders of magnitude enhanced charge carrier concentration (i.e., n≈ 1018 cm−3) and ≈ 6–9 times larger charge carrier mobilities (μ) than corresponding binary specimens. A kink at 200 K in ρT 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 (κph) mediated. The Debye-Callaway fits of κph 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., smaller y). The Balkanski-Klemens analysis of the temperature dependent Raman shifts indicated domination of three-phonon scattering mechanism agreeing with the lowered κph 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 κph. Te-doping of In-rich thiospinels (y ≈ 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 κph and the improvement of the TE performance are discussed.