Journal
DALTON TRANSACTIONS
Volume 43, Issue 27, Pages 10529-10540Publisher
ROYAL SOC CHEMISTRY
DOI: 10.1039/c4dt00336e
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Funding
- Deutsche Forschungsgemeinschaft [OE530/1-2]
- ESRF [HS-4363]
- European Union (European Social Fund)
- European Union (NFG Effiziente Energienutzung: Neue Konzepte und Materialien)
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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-Ge0.6Sn0.4Sb2Te4 and 9P-Ge1.3Sn0.7Sb2Te5. 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-Ge0.6Sn0.4Sb2Te4 was confirmed by high-resolution electron microscopy and electron diffraction. In contrast, electron diffraction patterns of 9P-Ge1.3Sn0.7Sb2Te5 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 = similar to 0.25 at 450 degrees C for both Ge0.6Sn0.4Sb2Te4 and Ge1.3Sn0.7Sb2Te5, which is higher than similar to 0.20 for unsubstituted stable layered Ge-Sb-Te compounds.
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