PREPARATION AND CRYSTAL-STRUCTURE OF CA5HG3 AND SR5CD3

Both the incongruently melting compounds, Ca5Hg3 and Sr5Cd3, have been synthesized from stoichiometric amounts of the pure elements. They crystallize 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 fragments 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 the only constituent moieties. These rhombic dodecahedra are built up by eight Ca (Sr) atoms and six Hg (Cd) atoms and are furthermore centered by an additional Ca (Sr) atom. Along [001] the rhombic dodecahedra share common vertices, but along [110] they are interconnected via common triangular faces. This kind of face sharing is responsible for the short distances obtained between the polyhedra, which leads to the occurence of the dumbbells mentioned above.