TbMgNi_4-<i>x</i>Co_<i>x</i>–(H,D)₂ System. I: Synthesis, Hydrogenation Properties, and Crystal and Electronic Structures

A series of cubic TbMgNi4-xCox (x = 0–4) pseudo-binary compounds have been synthesized by either a solid-state reaction or mechanical alloying with further annealing treatment. Their hydrogenation properties have been studied by pressure–composition–temperature (PCT) measurements, showing the formation of α-, β-, and γ-hydrides. The Co for Ni substitution yields a lowering of the equilibrium pressure and an increase of the hydrogen capacity. An improvement of the kinetics of hydrogen absorption was observed with an increasing Co content. β-TbMgNi4-xCoxH∼4 (x = 0–3) hydrides show an orthorhombic distortion (Pmn21 space group), whereas the β-TbMgCo4H3.3 hydride crystallizes in a monoclinic structure (Pm space group) derived from the orthorhombic structure. Sensitivity of the formation of the β-TbMgCo4H3.3 hydride to temperature has been observed in PCT curves to exist below 50 °C. γ-TbMgNi4-xCoxHy (x = 2–4, y > 5) hydrides preserve the parent cubic structure (F4̅3m space group) with a hydrogen-induced volume expansion of 17.9–22.3%. The deuterium for hydrogen substitution in the TbMgNiCo3–(H,D)2 system prevents fast desorption at room temperature and ambient pressure. The positions of deuterium atoms in the γ-TbMgCo4D6 deuteride were determined by neutron powder diffraction. Two types of interstitial sites were found to be occupied: 87% of deuterium atoms (D1 site) are located inside Tb2MgCo2 octahedra and the remaining 13% (D2 site) are inside Co4 tetrahedra. The increase of the Co content allows the improvement of the thermodynamic stability of TbMgNi4-xCox hydrides, as demonstrated by the lowering of the desorption plateau pressure, the increase of the desorption temperature, and the increase of the enthalpy of formation versus the Co content. This last experimental result is confirmed by first-principles calculations where electronic structures of intermetallic compounds and their cubic TbMgNi4-xCoxH6 hydrides are compared for x = 0, 2, and 4.