Y6Mg9Co2 and Y9Mg30Co2: Novel magnesium-rich compounds representing new structure types

Highlights: • New ternary intermetallics Y₆Mg₉Co₂ and Y₉Mg₃₀Co₂ were obtained. • Crystal chemistry of new Mg-rich intermetallics is analyzed in detail. • Ab initio calculations showed significant covalent Mg–Mg bonding in both structures. The crystal structures of the magnesium-rich Y₆Mg₉Co₂ and Y₉Mg₃₀Co₂ compounds have been determined from single crystal X-ray diffraction data. Both compounds represent new structure types: Y₆Mg₉Co₂ is orthorhombic, Pnnm, oP34, a = 8.7883 (2), b = 15.9669 (3), c = 5.9984 (2) Å; Y₉Mg₃₀Co₂ is hexagonal, P6₃/mmc, hP82, a = 10.1564 (3), c = 22.498 (1) Å. The crystal structures of these compounds are discussed in comparison with each other and with other magnesium containing compounds. Electronic structure calculations revealed that magnesium atoms form Mg₄-tetrahedra with significant covalent Mg–Mg interaction (-iCOHP = 0.732 eV for Y₆Mg₉Co₂ and 0.815 eV for Y₉Mg₃₀Co₂). The ELF and Bader charge analyses confirm that valence electrons localized around Mg atoms present a covalent bonding character.