LiAlTt (Tt = Si, Ge): Experimental and Theoretical Reinvestigation on the Thermoelectric Properties of 8-Valence-Electron Half-Heusler Compounds

Half-Heusler (HH) compounds with the valence electron count (VEC) of 18 have long been considered as excellent thermoelectric materials; however, for the 8-VEC analogues, few studies were carried out in the literature. In this work, two 8-VEC HH compounds, LiAlSi and LiAlGe, were reinvestigated as potential thermoelectric materials. The results indicated that both LiAlSi and LiAlGe possessed a high Seebeck coefficient near ambient temperature. Besides, LiAlSi is n-typed, which corresponds to a negative Seebeck coefficient of −260 μV·K–1 at 423 K, whereas for LiAlGe, p-type carriers were suggested with the Seebeck coefficient reaching 215 μV·K–1 at 473 K. Different from previous experiments, LiAlGe features a high thermal conductivity similar to LiAlSi, which primarily arises from the vibration of tetrel atoms according to the phonon structure calculations. Based on this understanding, the thermoelectric properties of such 8-VEC HH compounds were successfully optimized by configuring the solid solutions of LiAlSixGe1–x. As a result, a figure of merit of 0.43 was achieved in the material LiAlSi0.15Ge0.85 at 673 K, about 4 times higher than that of previous results for LiAlGe. This work indicates that the 8-VEC HH compounds are also very promising for thermoelectric consideration if the thermal conductivity can be significantly reduced.