Phase-pure crystalline Bi 2 Se 3 and Bi 2 Te 3 nanoparticles are formed in reactions of [C 4 C 1 Im] 3 [Bi 3 I 12 ] (C 4 C 1 Im = 1butyl-3-methylimidazolium) with [C 4 C 1 Pyr][ESiMe 3 ] (E = Se or Te; C 4 C 1 Pyr = 1-butyl-1-methylpyrrolidinium) in the ionic liquid (IL) [C 4 C 1 Im]I. The resulting crystalline tetradymite-type nanoparticles exhibit stoichiometric Bi:E (E = Se or Te) molar ratios (2:3). Because all synthetic steps were performed under strict inert gas conditions, the surfaces of the Bi 2 Se 3 and Bi 2 Te 3 nanoparticles are free of metal oxide species. As proven by infrared and X-ray photoelectron spectroscopy analyses, the nanoparticle surfaces reveal only minor organic contamination from solvent residues ([C 4 C 1 Im]I). The nanomaterials show high Seebeck coefficients of −124 μV K −1 (Bi 2 Se 3 ) and −155 μV K −1 (Bi 2 Te 3 ) and feature high electrical conductivities (328 and 946 S cm −1 , respectively) at the highest tested temperature (240 °C). The corresponding thermal conductivities (0.8 and 2.3 W m −1 K −1 , respectively, at 30 °C) are comparable to those of single crystals and recently reported ab initio calculations, which is in remarkable contrast to typical findings of nanograined bulk materials obtained from compacted nanoparticles. These findings emphasize the low level of impurities, surface contamination, and, in general, defects produced by the synthetic approach reported here. The figure of merit in the in-plane direction of the compacted pellets reached peak values 0.45 for Bi 2 Se 3 and 0.4 for Bi 2 Te 3 .

The synthesis of phase‐pure ternary solutions of tetradymite‐type materials (BixSb1−x)2Te3 (x=0.25; 0.50; 0.75) in an ionic liquid approach has been carried out. The nanoparticles are characterized by means of energy‐dispersive X‐ray spectroscopy (EDX), powder X‐ray diffraction (PXRD), scanning electron microscopy (SEM), and transmission electron microscopy. In addition, the role of different processing approaches on the thermoelectric properties ‐ Seebeck coefficient as well as electrical and thermal conductivity ‐ is demonstrated.