Layered germanium tin antimony tellurides: element distribution, nanostructures and thermoelectric properties

Abstract

In the system Ge–Sn–Sb–Te, there is a complete solid solution series between GeSb₂Te₄ and SnSb₂Te₄. As Sn₂Sb₂Te₅ does not exist, Sn can only partially replace Ge in Ge₂Sb₂Te₅; samples with 75% or more Sn are not homogeneous. The joint refinement of high-resolution synchrotron data measured at the K-absorption edges of Sn, Sb and Te combined with data measured at off-edge wavelengths unambiguously yields the element distribution in 21R-Ge_0.6Sn_0.4Sb₂Te₄ and 9P-Ge_1.3Sn_0.7Sb₂Te₅. In both cases, Sb predominantly concentrates on the position near the van der Waals gaps between distorted rocksalt-type slabs whereas Ge prefers the position in the middle of the slabs. No significant antisite disorder is present. Comparable trends can be found in related compounds; they are due to the single-side coordination of the Te atoms at the van der Waals gap, which can be compensated more effectively by Sb³⁺ due to its higher charge in comparison to Ge²⁺. The structure model of 21R-Ge_0.6Sn_0.4Sb₂Te₄ was confirmed by high-resolution electron microscopy and electron diffraction. In contrast, electron diffraction patterns of 9P-Ge_1.3Sn_0.7Sb₂Te₅ reveal a significant extent of stacking disorder as evidenced by diffuse streaks along the stacking direction. The Seebeck coefficient is unaffected by the Sn substitution but the thermal conductivity drops by a factor of 2 which results in a thermoelectric figure of merit ZT = ∼0.25 at 450 °C for both Ge_0.6Sn_0.4Sb₂Te₄ and Ge_1.3Sn_0.7Sb₂Te₅, which is higher than ∼0.20 for unsubstituted stable layered Ge–Sb–Te compounds.