On Ternary Intermetallic Aurides: CaAu 2 Al 2 , SrAu 2– x Al 2+ x and Ba 3 Au 5+ x Al 6– x

The intermetallic compound CaAu2Al2, and the members of the solid solutions SrAu2–xAl2+x (0 ≤ x ≤ 0.33) and Ba3Au5+xAl6–x (x = 0, 0.14, 0.49) were synthesized from the elements in sealed tantalum ampoules. The Ca compound crystallizes with the orthorhombic ThRu2P2 type structure, whereas the targeted SrAu2Al2 was found to form a solid solution according to SrAu2–xAl2+x. For the Ba system no “BaAu2Al2” was found, however, Ba3Au5+xAl6–x was discovered to crystallize in the monoclinic space group C2/c with its own structure type. The samples were investigated by powder X-ray diffraction and their crystal structures were refined on the basis of single-crystal X-ray diffraction data. All compounds were characterized furthermore by susceptibility measurements. The crystallographic aluminum sites of CaAu2Al2 and Ba3Au5Al6 can be differentiated by 27Al solid state NMR spectra on the basis of their different electric field gradients, in agreement with theoretical calculations. The electron transfer from the alkaline earth metals and the aluminum atoms onto the gold atoms was investigated by X-ray photoelectron spectroscopy (XPS) classifying these intermetallics as aurides, in full agreement with the calculated Bader charges.