New CeMgCo4 and Ce2MgCo9 compounds: Hydrogenation properties and crystal structure of hydrides

Two new ternary intermetallic compounds, CeMgCo4 (C15b pseudo-Laves phase, MgCu4Sn type) and Ce2MgCo9 (substitution derivative of PuNi3 type) were synthesized by mechanical alloying method. The structural and hydrogenation properties of these compounds were studied by X-ray diffraction and Pressure–Composition–Temperature measurements. Both compounds absorb hydrogen at room temperature and pressures below 10MPa forming hydrides with maximum compositions CeMgCo4H6 and Ce2MgCo9H12. Single plateau behavior was observed in P–C isotherm during hydrogen absorption/desorption by Ce2MgCo9 alloy. The CeMgCo4–H2 system is characterized by the presence of two absorption/desorption plateaus corresponding to formation of β-CeMgCo4H4 and γ-CeMgCo4H6 hydride phases. The structure of β-hydride CeMgCo4H(D)4 was determined from X-ray and neutron powder diffraction data. In this structure initial cubic symmetry of CeMgCo4 is preserved and hydrogen atoms fill only one type of interstitial sites, triangular MgCo2 faces. These positions are occupied by 70% and form octahedron around Mg atom with Mg–D bond distances 1.84Å. Crystal structure of the β-CeMgCo4D4.2 deuteride. Octahedra of D-sites around Mg atoms are shown. [Display omitted] ► Two new ternary compounds have been synthesized in the Ce–Mg–Co system. ► Below 100bar H2 CeMgCo4 and Ce2MgCo9 reversibly absorb hydrogen at room temperature. ► Crystal structure of cubic CeMgCo4D4.2 deuteride has been determined.