Alkaline earth-gold-aluminides: synthesis and structure of SrAu3Al2, SrAu2.83Al2.17, BaAu2.89Al2.11 and BaAu7.09Al5.91

New alkaline earth-gold-aluminides were synthesized from the elements in sealed tantalum or quartz ampoules in muffle furnaces at maximum annealing temperatures of 1325 K. The structures were refined from single crystal X-ray diffractometer data. SrAu3Al2 crystallizes in an ordered version of the LT-SrZn5 structure: Pnma, a = 1315.9(3), b = 549.0(1), c = 684.5(3) pm, wR2 = 0.0232, 930 F2 values, 35 variables. SrAu 2.83 Al 2.17 (a = 1065.0(2), b = 845.0(2), c = 548.1(1) pm, wR2 = 0.0416, 452 F2 values, 22 variables) and BaAu2.89Al2.11 (a = 1096.1(3), b = 835.7(3), c = 554.0(1) pm, wR2 = 0.0280, 501 F2 values, 22 variables) both adopt the BaZn5 type, space group Cmcm with Au/Al mixing on the 4c site. The gold and aluminum atoms in both types form three-dimensional networks of condensed tetrahedra with the strontium and barium atoms in large cavities. BaAu7.09Al5.91 is a new member of the NaZn13 type: Fm (3) over barc, a = 1257.6(2) pm, wR2 = 0.0267, 168 F2 values, 12 variables. Both the 96i and 8b sites show Au/Al mixing. The crystal chemical details are discussed.