Zn-rich Zincides of the Ternary System Ca-Mg-Zn: Synthesis, Crystal structure, Chemical Bonding

摘要

(In the course of a study on the role of magnesium in polar zincides of the heavier alkaline-earth elements, three intermetallic phases of the ternary system Ca-Mg-Zn were synthesized from melts of the elements and their structures were determined by means of single-crystal X-ray data. Starting from the binary zincide CaZn11, the phase width of the BaCd11-type structure reaches up to the fully ordered stoichiometric compound CaMgZn10 [tI48, space group I41/amd, a = 1082.66(6), c = 688.95(5) pm, Z = 4, R1 = 0.0239]. The new compound CaMgZn5 (oP28, space group Pnma, a = 867.48(3), b = 530.37(5), c = 1104.45(9) pm, Z = 4, R1 = 0.0385) crystallizes in the CeCu6-type structure, exhibits no Mg/Zn phase width and has no binary border equivalent in the system Ca-Mg-Zn. Similar to the situation in CaMgZn10), one M position of the aristotype has a slightly larger coordination sphere (CN = 14) and is accordingly occupied by the larger Mg atoms. The third phase, Ca2+xMg6-x-yZn15+y (hP92, space group P6(3)/mmc, a = 1476.00(5), c = 881.01(4) pm, Z = 4, R-1 = 0.0399 for Ca2.67Mg5.18Zn15.15) forms the hexagonal Sm3Mg13Zn30-type structure also known as -MgZnRE or S phase. A small phase width (x = 0-0.67; y = 0-0.58) is due to the slightly variable Ca or Zn content of the two Mg positions. The structure is described as an intergrowth of the hexagonal MgZn2 Laves phase and the CaZn2 structure (KHg2-type). All compounds exhibit strong Zn-Zn and polar Mg-Zn covalent bonds, which are visible in the calculated electron density maps. Their structures are thus herein described using the full space tilings of [Zn-4] and [MgZn3] tetrahedra, which are fused to polyanions consisting of tetrahedra stars, icosahedra segments etc. and the large (CN = 18-22) Ca cation coordination polyhedra. Pseudo bandgaps apparent in the tDOS are compatible with the narrow v.e./M ranges observed for other isotypic members of the three structure types.