The new complex barium mercuride Ba_(20)Hg _(103)and its ternary zinc and cadmium variants[Das neue komplexe barium-mercurid Ba _(20)Hg _(103)und seine tern?ren zink- und cadmium-varianten]

摘要

Ba _(20)Hg _(103), the next Hg-rich binary barium mercuride after BaHg _(11)and BaHg _6, was synthesized from melts of the elements, which were slowly cooled from 550 to 200 ?C. It forms a new complex cubic structure type(F4?3m, a = 2333.30(10)pm, Z = 4, R1 = 0.0651)with four Ba und 13 Hg positions as determined by means of single-crystal X-ray diffraction. The structure can be best described by decomposing it into four types(A to D)of space-filling distorted truncated octahedra(sodalite or β-cages)centered at the four sites with 4?3m point group symmetry of the space group. The cages C and D are centered by[Hg 8]tetrahedra stars, which are connected via[Hg(13)Hg _8]cubes to form a diamond-type network. The polyhedra B contain a section of the Laves phase MgCu _2(five[Hg _4]tetrahedra sharing corners). The smallest polyhedron A contains a truncated tetrahedron[HgHg _(12)]surrounded by a strongly covalently bonded mercury shell. The ternary cadmium derivative Ba _(20)Cd _4Hg _(99)(a = 2331.57(14)pm, R1 = 0.0465)is isotypic, whereas in the related zinc mercuride Ba _(20)Zn _5Hg _(99)(a = 2332.33(7)pm, R1 = 0.0436)one of the tetrahedra stars is filled by an extra zinc atom and is thus distorted into a[ZnHg _8]cube. This extra zinc atom causes the small change in the compounds formula. The four crystallographically different Ba cations are coordinated by 17 or 18 Hg atoms. The mercury atoms themselves are surrounded by 10 to 13, in one case 16, Hg/Ba neighbors. The shortest Hg-Hg distances(d ~(min)_(Hg-Hg)= 285.8 pm)are found for the strong covalent bonds in the shell around the sodalite cages A and the cubes around Hg(13). The bonding modes(Hg-Hg distances and overall coordinations numbers)of the different Hg atoms of Ba20Hg103 are discussed in comparison with those in the more Hg-rich compounds BaHg6 and BaHg11. According to their crystal-chemical properties, the Hg atoms in Hg-rich mercurides can be classified into four different groups, from mainly covalent to polar intermetallic and pure metallic, to finally cationic centers.