Phase equilibria in the Tb-Mg-Co system at 500 °C, crystal structure and hydrogenation properties of selected compounds

• The phase equilibria at 500 °°C in the Tb–Mg–Co system has been studied.
• The existence of two ternary compounds, Tb4Mg3Co2 and Tb4MgCo, was confirmed.
• The formation of two solid solutions, Tb1−xMgxCo3 and Tb1−xMgxCo2, was found.
• Hydrogen sorption–desorption properties have been studied for the selected alloys.

The isothermal section of the Tb–Mg–Co phase diagram at 500 °C has been built on the basis of XRD analysis of forty samples prepared by powder metallurgy. The existence of two ternary compounds Tb4Mg3Co2 and Tb4MgCo was confirmed. The formation of two solid solutions, Tb1−xMgxCo3 (0≤x≤0.4) and Tb1-−xMgxCo2 (0≤x≤0.6), was found for the first time. It is shown that Tb5Mg24 also dissolves a small amount of Co. Other binary compounds do not dissolve the third component. The Tb4MgCo and TbMgCo4 compounds form hydrides (12.7 and 5.3 at.H/f.u. capacity, respectively) that retain the original structure of metallic matrices. Upon thermal desorption the Tb4MgCoH12.7 hydride was stable up to 300 °C and disproportionated at higher temperature. Two other hydrides, Tb4Mg3Co2H∼4 and Tb2MgCo9H12, are unstable in air and decompose into the initial compounds.

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