Abstract
The new isotypic intermetallic phases Ba5MIII 5MIV (MIII = Al, Ga; MIV = Sn, Pb) have been synthesized from stoichiometric amounts of the elements at maximum temperatures of 900 to 1000 ◦C. They crystallize in the hexagonal space group P6̄m2 (Ba5Al5Sn: a = 605.05(8), c = 1109.0(2) pm, R1 = 0.0137; Ba5Ga5Sn: a = 599.45(5), c = 1086.00(7) pm, R1 = 0.0485; Ba5Al5Pb: a = 606.9(2), c = 1112.0(4) pm, R1 = 0.0409 and Ba5Ga5Pb: a = 601.76(7), c = 1091.51(13) pm, R1 = 0.0295), forming a new structure type. Similar to the Zintl phases Ba2MIV (Co2Si structure type, orthorhombic, space group Pnma; Ba2Sn: a = 861.52(14), b = 569.85(9), c = 1056.9(2) pm, R1 = 0.0217 and Ba2Pb: a = 865.12(13), b = 569.1(2), c = 1061.8(2) pm, R1 = 0.0470), these new ternary phases contain isolated MIV atoms (coordinated by 11 Ba atoms). In addition, sheets of 3- and 4-bonded Al/Ga atoms similar to those in Ba3Al5 are present. In accordance with this, a formal subdivision of Ba5MIII 5MIV into Ba3MIII 5 ・ Ba2MIV can be performed to describe the observed intergrowth or chemical twinning of two different binary intermetallics to give the new ternary compounds. Beyond structural aspects, also the nature of the chemical bonding (as studied by FP-LAPW calculations) in these new, non-electron precise compounds in the vicinity of the Zintl border can be interpreted in this vein.
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