Influencing the thermoelectric properties of germanium antimony tellurides (GST) by substitutions with In, Sn and Se

Kurzfassung

The nanostructure of bulk germanium antimony tellurides (GST materials) can be tuned by utilizing phase transitions between a cubic disordered rocksalt-type high-temperature phase and a layered trigonal phase, which is stable at ambient temperature. In the cubic phases, Te occupies the anion position and Ge, Sb and vacancies are randomly disordered at the cation position, whereas the layered trigonal phases consist of 2D extended rocksalt-type slabs interconnected by van der Waals gaps. Quenching the cubic high-temperature phases of (GeTe)nSb2Te3 (4  n  19) yields pseudocubic thermoelectric materials with finite intersecting defect layers and figures of merit (ZT) up to 1.3 at 720 K [Rosenthal et al., Chem. Mater. 2011, 23, 4349.]. Their thermoelectric properties depend on the GeTe content (n) and the synthesis conditions (temperature, annealing time, cooling rate). The substitution with In, Sn or Se is possible over a wide compositional range. Quenching results in pseudocubic solid solution series with promising thermoelectric properties. Doping bulk GST material with Se and In reduces the transition temperature between the thermodynamically stable trigonal phase and the cubic high-temperature phase. This affects the formation of nanostructures as diffusion rates determine the lateral extension of the defect layers that are formed upon quenching the high-temperature phase. In addition to their influence on the electronic band structure, real-structure phenomena and mixed occupancies enhance the phonon scattering and thus provide a way to tune the thermoelectric properties. Substituting Ge with Sn results in a more pronounced parquet-like nanostructure at low GeTe contents (e. g. n = 4) and an increased electrical conductivity. For these samples, the Seebeck coefficient is unaffected and the higher electrical conductivity is compensated by the thermal conductivity so that the ZT value is nearly unchanged compared to unsubstituted GST. Due to a pronounced increase of the Seebeck coefficient, the ZT value of In-substituted samples like Ge12SbInTe15 is higher than that of unsubstituted samples with comparable GeTe content up to 570 K. As a rule, the Seebeck coefficient increases and the thermal conductivity is reduced when Te is substituted by Se. Therefore the ZT value of Ge7Sb2Te8Se2, for instance, increases up to 1.2 at 720 K, 6 times higher compared to the unsubstituted sample.