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Materials with unusual homonuclear interactions between late main group elements that may be of use in the thermoelectric energy conversion are in the focus of this project. We reviewed that situation for thermoelectric power generation in a recent article in Chem. Mater., published in 2010. During our investigations of the last year of this PRF grant, we finalized our investigations of Mo₃Sb_7-xTe_x, Hf₃Cu₂Ge_4–xSb_1+x, and uncovered a new selenide-telluride, Ba₂Cu_4–xSe_yTe_5-y.

  While the formula of Ba₂Cu_4–xSe_yTe_5-y might imply this material to be a simple substitution variant of Ba₂Cu_4–xTe₅ (space group C2/c), it forms a new structure type, space group I4₁2₁2. The telluride was discovered at the beginning of this PRF project. Despite crystallizing in different crystal systems, the telluride and the selenide-telluride exhibit topologically equivalent structure motifs, namely, chains of Cu(Se,Te)₄ tetrahedra with a Cu atom cis/trans chain as well as an almost linear Te atom chain. This chain exhibits alternating Te–Te distances of 3.12 Å and 3.45 Å in case of the telluride, and noticeably shorter ones in case of the selenide-tellurides, depending on the Se content of the compound. The materials were characterized in our group via Seebeck and electrical conductivity measurements to be p-type semiconductors. These results were published this year in Inorg. Chem..

The new germanide-antimonide Hf₃Cu₂Ge_4–xSb_1+x also crystallizes in a new structure type, space group P4/nmm. This material exists only with a small phase range, and decomposed upon annealing below 900°C. As such, it is a high temperature phase, entropy-stabilized by mixed Ge/Sb positions.  Its crystal structure may be understood as an intergrowth of the ZrSiS and the NdTe₃ types. The apparently undistorted square planar layers are comprised of statistical mixtures of Ge and Sb atoms in an approximate 9 : 1 ratio. In accord with the undistorted square layers and the electronic structure calculation, Hf₃Cu₂Ge_4–xSb_1+x is a metallic conductor. No phase transition was detected down to 10 K.

The structure of Mo₃Sb_7-xTe_x contains Te₄ squares and Sb₈ cubes with weak Sb–Sb interactions. We showed that its thermoelectric properties can be optimized by adjusting the Sb/Te ratio (published 2008 in J. Appl. Phys) and adding different transition metals into the Sb8 cubes (published 2010 in J. Alloys Compd.). Thus far the best thermoelectric properties were achieved for Fe_0.05Mo₃Sb_5.4Te_1.6 and Ni_0.06Mo₃Sb_5.4Te_1.6.

These Mo₃Sb_7-xTe_x results comprised the main part of H. Xu's thesis, who successfully defended her Ph. D. thesis in September 2010. The Ba-Cu chalcogenides constitute the main results of O. Mayasree's work, who defended her M. Sc. thesis this September as well. As such, this PRF grant made a major contribution to my career as supervisor as well.