The Y–Mg–Co ternary system: alloys synthesis, phase diagram at 500 °C and crystal structure of the new compounds

Abstract

The ternary phase diagram of the Y–Mg–Co system have been studied at 500 °C by combining X-ray diffraction (XRD), energy dispersive X-ray (EDX) and wavelength dispersive X-ray (WDX). The existence of four ternary compounds Y4MgCo (Gd4RhIn-type), Y9Mg30Co2 (own structure type), Y6Mg9Co2 (own structure type) and Y9Mg4Co (Hf9Mo4B-type) was confirmed. For all of them homogeneity range has been defined. Two new ternary compounds were identified in this work: Y12MgCo5 (Ho12BiCo5-type, orthorhombic, Space Group Immm, a = 9.5078(5)–9.538(4) Å, b = 9.4741(4)–9.531(4) Å, c = 10.0217(5)–10.061(5) Å) and ∼Y52Mg3Co45 (unknown crystal structure). YCo3 and YCo2 compounds form extended solid solutions where maximum solubility of Mg is 8 and 16 at. %, respectively. The solubility of Mg in YCo2 leads to a structure change from MgCu2-type to SnMgCu4-type. It was also found that binary MgCo2 compound dissolve 4.9 at. % Y and YMg can dissolve up to 2.5 at. % of Co. New binary Y5Co6 compound was determined with two structures: α – low- and β – high-temperature. The α-Y5Co6 was indexed as monoclinic (a = 22.214 Å, b = 7.509 Å, c = 5.885 Å and β = 108.22°), while β-Y5Co6 crystalizes in the orthorhombic structure (space group Cmcm) with lattice parameters a = 4.104(1) Å, b = 10.246(1) Å and c = 20.336(3) Å. Detailed crystallographic data were collected for the Y5Mg0.07(1)Co5.93(1) and Y12MgCo5 single crystals. The metallic type bonding for Y12MgCo5 was confirmed by electronic structure calculations.