Novel Compounds in the Sc-rich Part of the Systems Sc-{Co,Pd,Pt}-Ga: Structure and Bonding

The present paper comprises a detailed structural analysis of two novel compounds in the Sc-Co-Ga system: τ3-Sc50Co13Ga3 (space group Fm-3; ε-Mg26-xAg7+x type), and τ4-Sc6Co1.73+x+yGa1-x (space group Immm; x=0.43; y=0.14; Ho6Co2Ga derivative type). Although alloys in the isothermal section are oxygen-free, the alloy of τ3, prepared for single crystal growth via partial melting in an alumina crucible, contained a minor amount of oxygen and can be considered as oxygen-stabilized Sc50Co13Ga3O1.1 (WDX-data). A search for homologous compounds in the systems Sc-{Pd,Pt}-Ga prompted the existence of two compounds Sc54{Pd,Pt,Ga}17, which from X-ray single crystal analyses proved both to be isotypic with the Hf54Os17-type structure (space group Immm). Whereas Sc54(Pd0.652Ga0.348)17 has a very limited homogeneity region at 850°C, Sc54(Pt1-xGax)17 exhibits at 850°C a large homogeneity region (0<x<0.28) extending from Ga-rich Sc76Pt17.2Ga6.8 Sc54(Pt0.72Ga0.28)17 to novel binary Sc54Pt17. Interestingly, against the usual trend of solution hardening, hardness, Young's modulus and fracture toughness all decrease with increasing Ga-content: this effect is less pronounced for Sc54(Pd1-xGax)17 than for Sc54(Pt1-xGax)17 and is in line with DFT calculations indicating a weakening of bonding with increasing Ga content, thereby overcompensating the solution hardening effect. DFT modeling showed a common bonding feature for all the studied compounds – a strong electron localization inside the Sc6 octahedra, stacking of which leads to the formation of Mackay icosahedra in Sc54(Pd1-xGax)17, Sc54(Pt1-xGax)17, and Sc50Co13Ga3. The band structure of Sc6Co2.25Ga0.625 and Sc50Co13Ga3 indicates their metallic behavior, while Sc54(Pd1-xGax)17 and Sc54(Pt1-xGax)17 show distinct semimetal features. All investigated compounds in this work show similar trends in Bader charge distribution with a positively charged Sc sublattice and a negatively charged sublattice formed by transition metals (Co, Pd, Pt) and Ga.