[Zurück]

V.V. Romaka, G. Rogl, G. Binder, H. Michor, Jiri Bursik, A. Grytsiv, G. Giester, P. Rogl:
"Phase relations, structure, properties and DFT study of compounds in the Sc-rich part of the systems Sc-{Mn,Fe,Co,Ni}-Ga";
Journal of Alloys and Compounds, 919 (2022), 165540; S. 1 - 27.

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 Sc_2+xNi_1-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(Co_1−xGa_x)₃Co and Sc₁₀(Ni_1−xGa_x)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: Sc_15−x−y□_yCo_3+x−zGa_1+z (unique structure type, space group Immm; x = 1.60, y = 0.58; 0 ≤z ≤ 0.82) and Sc_50+xCo_13−zGa_3+z (ε-Mg_26−xAg_7+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(T_1−xGa_x)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 Sc₅₄Fe_9.4Ga_7.6 and Sc₅₄Co_7.3Ga_9.7 are almost temperature independent paramagnets, Sc₅₄Mn_8.5Ga_8.5 exhibits an approximate Curie-Weiss behavior: χ₀ = 1.03 × 10⁻³ emu/mol; θ_p = 15.8 K, μ_eff = 2.23 μ_B/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(Co_1−xGa_x)₃Co (x = 0, 0.67, 1), Sc₁₀(Ni_1−xGa_x)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.

Scandium alloys and compounds, Phase diagrams, Crystal structure, Magnetization, Mechanical properties, Electronic band structure

http://dx.doi.org/10.1016/j.jallcom.2022.165540