K_2Hg_7 and Rb_2Hg_7, Two Examples of a New Structure Thpe for Intermetallic Compounds

摘要

K_2Hg_7 and Rb_2Hg_7 can be prepared from the elements as silver-metallic air sensitive microcrystalline samples. As shown by Rietveld refinement of powder data both solids crystallize isotypically in space group P3m1 (No. 164, Z = 1) with the lattice constants a = 671.75(2) pm, c = 641.33(2) pm (K_2Hg_7) and a = 684.26(2) pm, C = 657.74(1) pm (Rb_2Hg_7). K_2Hg_7 and Rb_2Hg_7 are the first examples for a new structure type of intermetallic compounds that can be regarded as hexagonal defect perowskite or filled Ni_3Sn structure in the sense of K_2Hg square Hg_6 (Rb_2Hg square Hg_6). Chains of face sharing Hg_6 octahedra surrounded by alkali metal atoms and running parallel to [001] are characteristical for the M_2Hg_7 structure. Every other octahedron is centered by an additional Hg atom leading to an alternating compression and elongation of the empty and filled octahedra, respectively. The formal character of this structure description is obvious because the shortest d_(Hg-Hg) approx= 300 pm do not occur inside but between neighbored chains of octahedra. Thus alternative structure descriptions based on mercury 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 atoms on the positions of a hexagonal close packing. These aspects are discussed in the paper together with systematical symmetry relations.