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

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, Tb_1−xMg_xCo₃ (0≤x≤0.4) and Tb_1-−xMg_xCo₂ (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 Tb₄MgCoH_12.7 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. - Highlights: • 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, Tb_1−xMg_xCo₃ and Tb_1−xMg_xCo₂, was found. • Hydrogen sorption–desorption properties have been studied for the selected alloys.