Revisiting some chalcogenides for thermoelectricity

Thermoelectric materials that are efficient well above ambient temperature are needed to convert waste-heat into electricity. Many thermoelectric oxides were investigated for this purpose, but their power factor (PF) values were too small (∼10^-4 W m^-1 K^-2) to yield a satisfactory figure of merit zT. Changing the anions from O^2- to S^2- and then to Se^2- is a way to increase the covalency. In this review, some examples of sulfides (binary Cr-S or derived from layered TiS₂) and an example of selenides, AgCrSe₂, have been selected to illustrate the characteristic features of their physical properties. The comparison of the only two semiconducting binary chromium sulfides and of a layered AgCrSe₂ selenide shows that the PF values are also in the same order of magnitude as those of transition metal oxides. In contrast, the PF values of the layered sulfides TiS₂ and Cu_0.1TiS₂ are higher, reaching ∼10^-3 W m^-1 K^-2. Apparently the magnetism related to the Cr-S network is detrimental for the PF when compared to the d⁰ character of the Ti⁴⁺ based sulfides. Finally, the very low PF in AgCrSe₂ (PF = 2.25 × 10^-4 W m¹ K^-2 at 700 K) is compensated by a very low thermal conductivity (κ = 0.2 W m^-1 K^-1 from the measured C_p) leading to the highest zT value among the reviewed compounds (zT_700K = 0.8). The existence of a glassy-like state for the Ag⁺ cations above 475 K is believed to be responsible for this result. This result demonstrates that the phonon engineering in open frameworks is a very interesting way to generate efficient thermoelectric materials.