2019/01/24 by Katharina Köhler, Caroline Röhr · 1 citation
Physics and Astronomy · Materials Science · #Rare-earth and actinide compounds #Hydrogen Storage and Materials #Magnesium Alloys: Properties and Applications
paper · doi:10.1002/zaac.201800429
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 CaZn 11 , the phase width of the BaCd 11 ‐type structure reaches up to the fully ordered stoichiometric compound CaMgZn 10 [ tI 48, space group I 4 1 / amd, a = 1082.66(6), c = 688.95(5) pm, Z = 4, R 1 = 0.0239]. The new compound CaMgZn 5 ( oP 28, space group Pnma, a = 867.48(3), b = 530.37(5), c = 1104.45(9) pm, Z = 4, R 1 = 0.0385) crystallizes in the CeCu 6 ‐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 CaMgZn 10 , 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, Ca 2+ x Mg 6– x – y Zn 15+ y ( hP 92, space group P 6 3 / mmc, a = 1476.00(5), c = 881.01(4) pm, Z = 4, R 1 = 0.0399 for Ca 2.67 Mg 5.18 Zn 15.15 ) forms the hexagonal Sm 3 Mg 13 Zn 30 ‐type structure also known as μ‐MgZn RE 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 MgZn 2 Laves phase and the CaZn 2 structure (KHg 2 ‐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 [MgZn 3 ] 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.