Sr2Au6Al3 and Eu2Au6Al3 - First Representatives of the Sr2Au6Zn3 Type with Aluminum Triangles

The aluminides Sr2Au6Al3 and Eu2Au6Al3 were synthesized by melting the elements in sealed tantalum tubes in a muffle or induction furnace. The samples were characterized by powder and single-crystal X-ray diffraction: Sr2Au6Zn3 type, R (3) over barc, a = 845.1(1), c = 2177.2(3) pm, wR2 = 0.0263, 520 F-2 values, 20 variables for Sr2Au6.18(1)Al2.82(1), and a = 838.0(1), c = 2177.1(7) pm, wR2 = 0.0276, 510 F-2 values, 19 variables for Eu2Au6Al3. The gold atoms build up diamond-related networks of slightly distorted tetrahedra in the stacking sequence of the 6R polytype (289 296 pm Au-Au in Eu2Au6Al3). The voids left by this network are filled in an ordered manner by strontium (europium) atoms and the rare motif of Al-3 triangles (286 pm Al-Al in Eu2Au6Al3). The Al-3 triangles in Sr2Au6.18(1)Al2.82(1) show a small degree of Al/Au mixing.