PREPARATION AND CRYSTAL-STRUCTURE OF CA5H G3 AND SR5CD3

Both the incongruently melting compounds, Ca5Hg3 and Sr5Cd3, have been synthesized from stoichiometric amounts of the pure elements. They cr ystallize with the Cr5B3 type of structure: space group I4/mcm, Z = 4; Ca5Hg3 (Sr5Cd3): a = 818.9(1) (871.7(1)) pm, c = 1470.1(3) (1660.1(3) )pm, c/a = 1.80 (1.90), R = 2.33% (2.97%). The most remarkable fragmen ts are dumbbells X(2), which have interatomic distances only slightly longer than the sum of Pauling's covalent radii: Hg-Hg (Cd-Cd) = 306 ( 298) pm. The structure can be constructed by rhombic dodecahedra as th e only constituent moieties. These rhombic dodecahedra are built up by eight Ca (Sr) atoms and six Hg (Cd) atoms and are furthermore centere d by an additional Ca (Sr) atom. Along [001] the rhombic dodecahedra s hare common vertices, but along [110] they are interconnected via comm on triangular faces. This kind of face sharing is responsible for the short distances obtained between the polyhedra, which leads to the occ urence of the dumbbells mentioned above.