Thermoelectric properties of metastable Ge/Sb/Te and Ge/Bi/Te compounds

Abstract

For both (GeTe)n(Sb2Te3) (n = 3, 4.5, 7, 12, 19) and GeBi2Te4, samples with different nanostructures can be obtained. The nanostructure of (GeTe)n(Sb2Te3) depends on the GeTe content n as well as on the thermal treatment of the samples. Low GeTe contents (n < 4.5) lead to a high concentration of vacancies, which arrange in parallel defect layers and form van der Waals gaps upon annealing. Higher GeTe contents, i.e. less defects, lead to pseudocubic structures with finite intersecting vacancy planes. Annealing such samples at 450 °C leads to diffusion, yielding parallel gaps between distorted rocksalt-type blocks. ZT values of up to 1.3 have been obtained for quenched samples with n = 12 or 19, whereas the maximum ZT = 0.3 was reached for annealed samples with parallel van der Waals gaps. For GeBi2Te4, various nanostructures can be introduced by crystallization under high pressure (12 GPa) and different thermal treatment. Small particles (< 10 nm) with a high concentration of grain boundaries can be obtained by quenching the melt. A broad range of particle sizes and orientations is yielded by quenching the solid. However, samples which were slowly cooled from the melt possess a nanostructure like (GeTe)4.5(Sb2Te3). Nanostructures with high concentrations of domain or grain boundaries do not exhibit good thermoelectric properties, because the boundaries increase the electrical resistivity more than they reduce the thermal conductivity.