In situ neutron-diffraction study of deuterium desorption. from LaNi5+x(x∝1) alloy

Structural changes during deuterium desorption from highly over-stoichiometric LaNi5.95 alloy have been monitored by in situ powder neutron diffraction and related to its pressure-composition-isotherm (PCI) at room temperature. The PCI curve exhibits no clear plateau pressure and a maximum capacity of 4.9 D-atoms per unit cell (D/u.c.) at 10 MPa. However, structural investigations reveal a phase transition from β-deuteride to α-solid solution with a reduced α+β coexistence region, which extends from 1.4 to 3.5 D/u.c. This transition exhibits variable α- and β-lattice volumes and involves a minor lattice expansion of 3.5 vol. %. It was found that the required volume expansion to host D-atoms in the β-phase is as high as 4.8 A3/D, which could be related to the reduced H-storage capacity in over-stoichiometric LaNi5+x alloys. (orig.)