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

The ternary phase diagram of the Y-Mg-Co system have been studied at 500 degrees 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) angstrom, b = 9.4741(4)-9.531(4) angstrom, c = 10.0217(5) -10.061(5) angstrom) and similar to 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: alpha - low- and beta - high-temperature. The alpha-Y5Co6 was indexed as monoclinic (a= 22.214 angstrom, b = 7.509 angstrom, c = 5.885 angstrom and beta= 108.22 degrees), while beta-Y5Co6 crystalizes in the orthorhombic structure (space group Cmcm) with lattice parameters a = 4.104(1) angstrom, b = 10.246(1) angstrom and c = 20.336(3) angstrom. 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. (C) 2019 Elsevier B.V. All rights reserved.