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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 polyselenides and polytellurides in a recent article, published in 2009. During our exploratory investigations, we uncovered two new materials with such interactions, Ba₂Cu_4–xTe₅ and Hf₃Cu₂Ge_4–xSb_1+x.

The new telluride Ba₂Cu_4–xTe₅ forms a new structure type, space group C2/c. A special structural motif is formed by its Te atoms, namely an almost linear chain with alternating interatomic distances of 3.12 Å and 3.45 Å. Hence, that chain is distorted compared to an equidistant Te atom chain, as found in Ca_0.5K₄Te₃. A closer inspection of the thermal expansion parameters revealed evidence for split sites within the Te atom chain of Ba₂Cu_4–xTe₅, leading to other Te–Te distances of 2.87 Å and 3.02 Å instead of the 3.12 Å, as well as 3.53 Å and 3.67 Å instead of the 3.45 Å. While no long range order was detected, such distortions strengthen the shorter interactions while weakening the longer interactions.

Our band structure calculations indicate that Ba₂Cu_4–xTe₅ should be metallic, when its Te atom chain is not distorted. Experimentally we determined semiconducting properties with a high Seebeck coefficient, thus confirming that the chain is indeed distorted as postulated based on the thermal expansion parameters.

The new germanide-antimonide Hf₃Cu₂Ge_4–xSb_1+x also crystallizes in a new structure type, space group P4/nmm. Its structural motifs are reminiscent of the ZrSiS type, adopted by HfGeSb published by our group in 2002, and the HfCuAs₂ type. Therein, apparently undistorted square planar layers are formed by statistical mixtures of Ge and Sb atoms in an approximate 9 : 1 ratio, noting that we found no evidence for a significant phase range. The bond distances of 2.69 Å within that square layer are equal to the bonds of the square planar Ge layer in HfGeSb, indicative of hypervalent Ge–Ge interactions. Such undistorted nets cause metallic conduction, as found for Hf₃Cu₂Ge_4–xSb_1+x as well.