Synchrotron diffraction studies and thermodynamics of hydrogen absorption–desorption processes in La 0.5Ce 0.5Ni 4Co

Abstract

A hexagonal CaCu 5-type La 0.5Ce 0.5Ni 4Co intermetallic alloy was studied as H storage material in as cast condition using PCT and in situ Synchrotron XRD studies performed between 263 and 353 K. A significant shrinking of the unit cells takes place on a substitution of La with Ce. SR XRD showed the formation of two hexagonal hydrides, a γ-trihydride La 0.5Ce 0.5Ni 4CoH 3, and a saturated β-hexahydride La 0.5Ce 0.5Ni 4CoH ∼6. In addition, an α-H solid solution was experimentally observed. Hysteresis of hydrogen absorption and desorption between the flat single plateaus isotherms measured during the PCT measurements was rather small, with no indication of the formation of the thermodynamically stable γ-(La,Ce)(Ni,Co) 5H 3. The SR XRD revealed that (a) the relative abundance of the γ-hydride is higher during the desorption compared to the absorption; its relative amount reaches 50 wt.% in maximum; (b) anisotropic strains in all constituent phases during both hydrogen absorption and desorption are not developed; (c) the pressure of the formation of the γ-phase was found to be strongly temperature-dependent. The formation of large amounts of metastable γ-(La,Ce)(Ni,Co) 5H 3 hydride in quasiequilibrium experimental conditions applied during the SR XRD experiments and during reaching the thermodynamic equilibrium between the constituent phases can be responsible for the reduced hysteresis of the PCT isotherms and absence of noticeable anisotropic strains in the constituent phases.