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.

Increasing Seebeck Coefficients and Thermoelectric Performance of Sn/Sb/Te and Ge/Sb/Te Materials by Cd Doping

“…[ 7 ] The most signifi cant differences concern the bonds near the van der Waals gap (A2 C2; 2.9491(7) Å vs 2.939 Å in 21 R -GeSb 2 Te 4 ) and interatomic distances across the van der Waals gap (3.7233(10) Å vs 3.749 Å in 21 R -GeSb 2 Te 4 ). [ 18 ] and 21 R -GeSb 2 Te 4 ; [ 17 ] two slabs around a van der Waals gap are shown.…”

“…With this method, the element distribution on the different Wyckoff positions of layered structures can be determined by joint multidataset refi nements. [ 18,25,28 ] The corresponding results can also serve as models for local structure elements in disordered quenched compounds. As the element distribution in stable layered compounds may serve as a model for the local surrounding of the van der Waals gaps in metastable quenched materials, 21 R -Cd 0.2 Ge 0.8 Sb 2 Te 4 and 21 R -Cd 0.2 Sn 0.8 Sb 2 Te 4 , which represent the maximum degree of Cd substitution, were investigated using resonant X-ray diffraction at the K absorption edges of Cd, Sn, Sb, and Te.…”

“…Compounds of the series (Ge 1x Sn x )Sb 2 Te 4 (0 ≤ x ≤ 1) or (Ge 1x Sn x ) 2 Sb 2 Te 5 (0 ≤ x ≤ 0.5) form layered structures (e.g., 21 R -GeSb 2 Te 4 type [ 17 ] with slightly improved thermoelectric properties due to lower κ p compared to pure GST materials. [ 18 ] Quenched GST samples containing Sn exhibit more pronounced nanostructures than unsubstituted ones. Quenched pseudocubic solid solutions (Ge 1x Sn x Te) n Sb 2 Te 3 ( n = 4, 7, 12; 0 ≤ x ≤ 1) were investigated concerning nanostructures and thermoelectric properties.…”

“…This seems intriguing in GeTerich GST and SnTe-rich SST compounds ( n ≥ 7), which exhibit higher Seebeck coeffi cients S than compounds with lower n and especially long-range ordered layered ones. [ 12,18,19 ] The structure elucidation of many of the compounds mentioned above is often not unequivocal due to the low scattering contrast of elements with similar electron count (e.g., Cd, Sn, Sb and Te). This problem can be overcome by resonant X-ray scattering, i.e., using anomalous dispersion at K absorption edges of the elements involved.…”

Synthesis of SnSb2Te4 Microplatelets by High-energy Ball Milling

“…Among several alloys with AB 2 X 4 (A= Ge, Pb, Sn; B = Sb, Bi; and X = Se, Te) composition, SnSb 2 Te 4 looks as a promising candidate for various important applications such as phase-change material for non-volatile memory applications or efficient thermoelectric material [1][2][3][4] . This compound crystallizes in a layered hexagonal lattice with its large unit cell composed of alternating layers of Sn, Sb or Te atoms 5 .…”