dc.description.abstract
Phase relations have been derived for the Sc-rich parts of the ternary systems Sc-T-Ga at 850 °C (T = Mn, Fe, Co, Ni) as well as for the corresponding regions of the pertinent binary systems. "Sc4Mn" was not observed, but hexagonal Sc2Ga (space group P63mc, unique type) is a binary phase (at 29-31 at% Ga) with a peritectic decomposition temperature (at 1075 °C) close to the corresponding eutectic with scandium at 1065 °C and 17.5 at% Ga. Electron microprobe analyses (EPMA) revealed very limited solid solubilities at 850 °C of T-elements in Sc (<2 at% T). Although the maximal solubility of Ga in Sc at 850 °C is about 1 at% Ga, it renders the Sc-phase brittle. An X-ray single crystal structure analysis and TEM investigation defined Sc2+xNi1-x (x = 0.15) as a highly disordered Ti2Ni-type related to the type of disorder known for Dy5Pd2. Whereas the isothermal ternary sections for Sc-{Mn,Fe}-Ga are characterized by one ternary compound, Sc54{Mn,Fe,Ga}17 with the orthorhombic Hf54Os17-type (space group Immm), the systems with Co, Ni contain a so-called kappa (κ) phase, Sc10(Co1−xGax)3Co and Sc10(Ni1−xGax)3Ni. Both phases are occupation variants of the κ-Hf9Mo4B-type (space group P63/mmc). Besides that, the Sc-Co-Ga system at 850 °C comprises also a hitherto unknown representative of the Hf54Os17-type as well as two further ternary solid solution phases: Sc15−x−y□yCo3+x−zGa1+z (unique structure type, space group Immm; x = 1.60, y = 0.58; 0 ≤z ≤ 0.82) and Sc50+xCo13−zGa3+z (ε-Mg26−xAg7+x type, space group Fm-3; x = 0.84; 0 ≤z ≤ 1). For all compounds the crystal structures were derived from X-ray single crystal studies, some even at several compositions. The characteristic features of all structure types are non-regular Sc-icosahedra centered by (T,Ga) atoms. Whereas κ-Sc10(T1−xGax)3T phases (T = Co,Ni) form a typical network of corner-connected empty octahedra Sc6, which encompass Sc-centered icosahedra Sc6Ga6 and T-centered trigonal prisms T[Sc6], the Sc54(T,Ga)17 phases are made of (T,Ga)-centered defect icosahedron-clusters (T = Mn, Fe, Co). The ternary compounds exhibit remarkable large homogeneity regions mainly at practically constant Sc-contents i.e. deviating only slightly from T/Ga exchange pointing towards Sc-richer limits: at 850 °C from 12 to 14.6 at% Ga for the Co-κ-phase and from 2.1 to 15.4 at% Ga for the Ni-κ-phase; from about 6-12 at% for the phases Sc54{Mn,Co,Ga}17, but from 2.8 to 12.3 at% Ga in the case of Sc54{Fe,Ga}17. It should be mentioned that Sc54{Fe,Ga}17as well as κ-Sc10(Ni,Ga)4 approach the corresponding binary system almost joining with binary cubic Sc29Fe6 or at least indicating a vanishing instability for the binary hypothetical kappa phase "Sc10Ni4". Physical properties have been studied for the compounds richest in Sc: specific heat and electrical conductivity measurements infer metallic behavior for the Sc54(T,Ga)17 phases. Whereas Sc54Fe9.4Ga7.6 and Sc54Co7.3Ga9.7 are almost temperature independent paramagnets, Sc54Mn8.5Ga8.5 exhibits an approximate Curie-Weiss behavior: χ0 = 1.03 × 10−3 emu/mol; θp = 15.8 K, μeff = 2.23 μB/Mn. At low temperatures it reveals a weakly interacting spin glass system with a spin-freezing anomaly at around 8 K. Hardness and elastic moduli are in the range of typically polar intermetallics. Chemical bonding and charge transfer is elucidated from DFT calculations and eDOS for the kappa phases, Sc10(Co1−xGax)3Co (x = 0, 0.67, 1), Sc10(Ni1−xGax)3Ni (x = 0.67), as well as for the compounds with selected composition Sc54{Mn,Fe,Co}10Ga7 and showed high electron localization inside the empty Sc6 octahedra for all studied compounds and related kappa phases, which may attract small and highly electronegative elements.
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