K2Hg7 and Rb2Hg7, two examples of a new structure type for intermetallic compounds

K2Hg7 and Rb2Hg7 can be prepared from the elements as silver-metallic air s ensitive microcrystalline samples. As shown by Rietveld refinement of powde r data both solids crystallize isotypically in space group P (3) over bar m l (No. 164, Z = 1) with the lattice constants a = 671.75(2) pm, c = 641.33( 2) pm (K2Hg7) and a = 684.26(2) pm, c = 657.74(1) pm (Rb2Hg7). K2Hg7 and Rb 2Hg7 are the first examples for a new structure type of intermetallic compo unds that can be regarded as hexagonal defect perowskite or filled Ni3Sn st ructure in the sense of K2Hg square Hg-6 (Rb2Hg square Hg-6). Chains of fac e sharing Hg-6 octahedra surrounded by alkali metal atoms and running paral lel to [001] are characteristical for the M2Hg7 structure. Every other octa hedron is centered by an additional Hg atom leading to an alternating compr ession and elongation of the empty and filled octahedra, respectively. The formal character of this structure description is obvious because the short est d(Hg-Hg) approximate to 300 pm do not occur inside but between neighbor ed chains of octahedra. Thus alternative structure descriptions based on me rcury double layers or distorted Kagome nets are possible too, The same is true for the assumption of an ordered distribution of Hg and alkali metal a toms on the positions of a hexagonal close packing. These aspects are discu ssed in the paper together with systematical symmetry relations.