Deuteration properties of CaNi5-xCux system
Intermetallic compounds with nominal formula CaNi5-xCux (x = 0, 1, 2.5) have been prepared in order to investigate their hydrogenation properties. The samples were obtained by arc-melting and were deuterated in a Sieverts reactor. For x=0 and 1, we have found that the fast kinetics and the different shape of the curve (non sigmoidal) in the second absorption process indicate an improvement of the hydrogen absorption due to the activation of the alloys. The deuterium desorption spectra are similar for x = 0 and 1 whereas for x=2.5 the desorption ranges a broader temperature interval (similar to 100-350 degrees C) indicating a certain degree of chemical inhomogeneity or amorphization intrinsic to the parent sample or induced by the deuterium absorption. The formed deuterides were passivated in the presence of air in order to carry out a neutron diffraction study, allowing us to determine the deuterium positions in the samples. While in CaNi4CuDy the deuterium is randomly distributed over seven different positions, in CaNi5Dy the deuterium only occupies five of them. This wider distribution in CaNi4CuDy can explain its higher stability, and therefore, its higher desorption temperature for deuterium.
- [Show abstract] [Hide abstract] ABSTRACT: Density functional theory based electronic structure calculations which included crystal structure optimizations were performed for CaNi5Hx (x = 0.0, 0.25, 0.5, 1.0) compounds. Only hydrides with one H atom per unit cell were considered in the present work. Site preference of hydrogen atoms and energetics of hydride formation were investigated. It was found that hydrogen atoms prefer, in notation of space group P6/mmm, 12n sites and that 3f sites are the second most preferred sites what is compatible with previous neutron diffraction studies. Enthalpy of formation of α solid solution and for α → β reaction were calculated and compared with available experimental data and fair agreement was found between calculated and experimental values of enthalpies of formation.0Comments 2Citations