Structures and physical properties of new semiconducting gold and copper polytellurides: Ba7Au2Te14 and Ba6.76Cu2.42Te14

The title compounds were prepared from the elements between 600 and 800 degrees C in evacuated silica tubes. Both tellurides, Ba7Au2Te14 and Ba6.76Cu2.42Te14, form ternary variants of the NaBa6Cu3Te14 type, space group P63/mcm, with a = 14.2593(7) A, c = 9.2726(8) A, and V = 1632.8(2) A3 (Z = 2) for Ba7Au2Te14 and a = 14.1332(4) A, c = 9.2108(6) A, and V = 1593.3(1) A3 (Z = 2) for Ba6.76Cu2.42Te14. The Na site is filled with a Ba atom (deficient in case of the Cu telluride) and the Cu site with 66.5(3)% Au and 61.7(8)% Cu. An additional site is filled with 9.5(7)% Cu in the structure of Ba6.76Cu2.42Te14. These structures are comprised of bent Te32- units and AuTe4/CuTe4 tetrahedra, forming channels filled with Ba cations. The BaTe9 polyhedra are connecting the channels to a three-dimensional structure. According to the formulations (Ba2+)7(Au+)2(Te32-)3(Te2-)5 and (Ba2+)6.76(Cu+)2.42(Te32-)3(Te2-)5, the materials are electron-precise with 16 positive charges equalizing the 16 negative charges. Correspondingly, both tellurides are semiconductors, as experimentally confirmed, with calculated band gaps of 0.7 and 1.0 eV, respectively.