46766-AC10
Unusual As–As and Sb–Sb Bonding in Thermoelectric Materials
In the focus of this project are materials with unusual Sb–Sb, As–As, or Te–Te interactions that may be of use in the thermoelectric energy conversion. To that end, we analyzed the impact of such interactions onto the thermoelectric properties of b-Zn4Sb3, LaRu4Sb12, Yb14MnSb11, and Mo3Sb5Te2, and published our findings in J. Comput. Chem. 29, 2134 (2008). The structure of b-Zn4Sb3 comprises linear chains of Sb atoms with alternating interatomic distances of 2.8 Å and 3.4 Å. The short interaction is reflected in sharp peaks of the densities of states both above and below the band gap, while the longer interaction occurs with a strong contribution just below the gap. Because the Seebeck coefficient is proportional to the first derivate of the densities of states, DOS (cf. Mott equation), both of these interactions strongly contribute to the size of the Seebeck coefficient, which in turn is responsible for the large figure-of-merit, ZT, of b-Zn4Sb3. Its high ZT also originates from its low thermal conductivity, which relates to the disorder of the Zn sites and the fluctuations within the Sb atom chain.
Similarly, the Sb–Sb bonds of the planar Sb4 ring of the skutterudites cause such peaks in the DOS above and below the Fermi level as well, as do the hypervalent interactions of the linear Sb37– unit of Yb14MnSb11 in the region above the gap. Finally, the same is true for the short Sb–Sb bond of 2.9 Å and the longer contacts along the cube edges of 3.1 Å of Mo3Sb5Te2.
The unique T net found in the binary antimonide Hf5Sb9 was further analyzed as well (currently in print in the journal Z. Anorg. All. Chem.). Hf5Sb9 is metastable below 1020°C, and decomposes slowly into HfSb2 and Hf at intermediate temperatures. Gaussian calculations of the molecular fragment Sb816– revealed that the idealized T variant, i.e. all bond angles being 90°, would be thermodynamically preferred, but is not realized because of space constraints in the actual Hf5Sb9 structure. The slightly puckered T net of Sb atoms itself was calculated to be metallic, and Hf d orbitals contribute to the metallic character as well. Our physical property measurements confirmed its metallic character as predicted by our electronic structure calculations. No phase transition was detected in the differential scanning calorimetry analysis of Hf5Sb9. Substitution attempts with other transition metal atoms in place of Hf were not successful.
Efforts to modify the electron count and hence the geometry and the electronic structure of the Sb net by partial Te replacements yielded a new material, HfSb2–xTex with x < 1 (unpublished results). HfSb2–xTex crystallizes in the OsGe2 type, and is isostructural with HfMoSb4, TiMoAs4, and TiMoSb4 discovered earlier in our research group (Inorg. Chem. 46, 1459 (2007)). This structure contains a puckered Sb layer consisting of Sb6 rings in the chair conformation. The rings exhibit a short Sb–Sb bond of 2.8 Å, a typical single bond, and an intermediate bond of 3.1 Å, the latter comparable to the hypervalent Sb–Sb bonds of 3.0 Å in the T net of Hf5Sb9. Next steps will include phase width and physical property determination, as well as substitution attempts.
During our parallel running attempts to modify the electronic structure of LaTe2, another material was discovered, namely La8–xTaxTe15. La8–xTaxTe15 (Gd8Se15 type) comprises a distorted and diluted square net of Te atoms. Our preliminary results indicate that La8–xTaxTe15 is semiconducting for small Ta contents, while LaTe2 was reported to be metallic by Stöwe et al. in the year 2000. This part of the project will also be expanded, e.g. investigating the phase range and the role of the small Ta content for the structure change. Thermoelectric properties will be determined in dependence of x, the Ta concentration.
The theoretical work on the Sb–Sb interactions of thermoelectric antimonides as well as of Hf5Sb9 was a major part of Dr. J. Xu's Ph.D. thesis, who defended it successfully in July 2008. The discovery of HfSb2–xTex will propel H. Xu's Ph. D. thesis significantly, likely enabling her to finish on time.
