Enhancing the Thermoelectric Properties of Germanium Antimony Tellurides by Substitution with Selenium in Compounds Ge_nSb₂(Te_1–xSe_x)_n+3 (0 ≤ x ≤ 0.5; n ≥ 7)

Quenched pseudocubic germanium antimony tellurides (GST compounds) exhibit promising thermoelectric properties. These are related to the nanostructures which can be influenced by varying the composition and the thermal treatment. The substitution of Te by Se in bulk samples of Ge n Sb 2 Te n +3 with high thermoelectric figures of merit ( ZT ) is possible over a wide compositional range. This results in solid solution series Ge n Sb 2 (Te 1– x Se x ) n +3 with 0 < x < 0.75 for n ≥ 7. Se substitution reduces the average lateral extension of the defect layers in quenched samples. This is a consequence of the reduced mobility during the quenching process due to the lower cubic to trigonal phase transition temperatures of Se-substituted samples. Most pronounced for n = 7, Se doping increases the transition temperatures between the nanostructured (pseudo)cubic modification of quenched samples and their layered trigonal phase. This increases the temperature ranges in which the materials can be employed without altering their nanostructures and properties. When Se is introduced, the Seebeck coefficient increases and the thermal conductivity decreases. The ZT value of Ge 7 Sb 2 Te 8 Se 2, for instance, increases up to 1.2 at 425 °C, which is 6 times higher than that of Ge 7 Sb 2 Te 10 . Similarly, the ZT value of Ge 12 Sb 2 (Te 1– x Se x ) 15 increases up to a factor of 2 for x = 0.2 at temperatures below 400 °C. The most promising thermoelectric properties ( ZT = 1.2 at 425 °C for n = 7 and ZT = 1.1 at 350 °C for n = 12) are observed for x = 0.2 whereas higher Se substitution rates result in a less pronounced effect. The average structures were determined by powder as well as single crystal X-ray diffraction (SCXRD). The real structure of quenched Ge n Sb 2 (Te 1– x Se x ) n +3 (0 ≤ x ≤ 0.5; n ≥ 7) bulk material was investigated by high-resolution electron microscopy (HRTEM) with respect to the degree of substitution ( x ) and correlated with the resulting changes of the thermoelectric properties. The decrease of the lateral extension of the defect layers with increasing Se content as found by HRTEM and electron diffraction is confirmed by SCXRD data for Ge ∼5 Sb 2 (Te 0.13 Se 0.87 ) ∼8.