title: 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(0.6)Sn(0.4)Sb2Te4 and 9P-Ge(1.3)Sn(0.7)Sb2Te5. 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 Sb(3+) due to its higher charge in comparison to Ge(2+). The structure model of 21R-Ge(0.6)Sn(0.4)Sb2Te4 was confirmed by high-resolution electron microscopy and electron diffraction. In contrast, electron diffraction patterns of 9P-Ge(1.3)Sn(0.7)Sb2Te5 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(0.6)Sn(0.4)Sb2Te4 and Ge(1.3)Sn(0.7)Sb2Te5, which is higher than ~0.20 for unsubstituted stable layered Ge-Sb-Te compounds.
authors.affiliation: Leipzig University, Faculty of Chemistry and Mineralogy, Scharnhorststr. 20, 04275 Leipzig, Germany. oliver.oeckler@gmx.de.
authors.forename: Simon
authors.initials: S
authors.lastname: Welzmiller
authors.forename: Tobias
authors.initials: T
authors.lastname: Rosenthal
authors.forename: Pirmin
authors.initials: P
authors.lastname: Ganter
authors.forename: Lukas
authors.initials: L
authors.lastname: Neudert
authors.forename: Felix
authors.initials: F
authors.lastname: Fahrnbauer
authors.forename: Philipp
authors.initials: P
authors.lastname: Urban
authors.forename: Christian
authors.initials: C
authors.lastname: Stiewe
authors.forename: Johannes
authors.initials: J
authors.lastname: de Boor
authors.forename: Oliver
authors.initials: O
authors.lastname: Oeckler