The Research On Intermediate Phase Hydrides And The Behavior Of Helium Atoms In LaNi₅-based Hydrogen Storage Material

The density functional theory and the full-potential linearized augmentedplane wave method with the generalized gradient approximation (GGA) are usedto optimize the cell and internal parameters of intermediate phase hydridesLaNi₅H_x(x=2, 3, 4, 5), and the equilibrium structure, density of states,chargedensity structure and the occupying rule of hydrogen atoms in the host alloy aresuccessfully obtained. Furthermore, the seven models of LaNi₅He have beenoptimized to find out the stable interstitial sites of Helium atom. And the structuresof hydrides LaNi₅H_5.5,5He_0.5,1, LaNi₅H_6.5,6He_0.5,1 and the related substitutedderivatives LaNi_4.5Al_0.5H_6,5.5,5He_0,0.5,1 are also investigated to seek after thebehavior of helium atoms in the alloy in detail. Finally, comparing withexperimental data, the results indicate that our calculated data are in wellagreement with the corresponding experimental data.From optimizing the cell parameters and internal coordinates for severalmodels, the stable structure and electronic properties of LaNi₅H_x(x=2, 3, 4, 5)could be successfully worked out. And the occupying rule of H atoms in the unitcell is well summarized, the first H atom primarily locates in the basal plane (12nsite); then the second H atom would prefer to occupy the middle plane site (6msite); when the number of H atoms increase fiom 3 to 5, they would occupy theotherl2n and 6m sites by turns. From the plots of density of states and electrondensities, it is shown that the hydrogen atoms make strong chemical bond with Niatoms rather than La atoms. Moreover the interactions between La atom and Ni atom further bring down gradually with the increase of hydrogen atoms.The micro-arrangement of Helium atom for tritium aging of LaNi₅He hasbeen modeled for theoretical study. The energy curves among 1b-6m-2d, 4h-2d-4h,6m-12o-12n and 12n-3f-12n interstitial sites for LaNi₅He are worked outtheoretically. The results indicate that 1b site is the most stable location among theseven nonequivalent interstices. The potential barrier doesn't exist when Heliumatoms migrate from 6m site to 1b site, but there is the potential barrier whichheight is 1.55 eV between 2d site and 1b site. And Helium atoms which lie in the12n sites could easily get across 12o sites and get to the 6m sites finally. Heliumatoms can freely migrate between the 4h sites. If only getting through a 13.6 eVpotential barrier, Helium atoms are able to directly migrate between the 12n sites.Finally, the density of state (DOS), charge density (CD) and potential distributiongraph for characteristic interstitial LaNi₅He and LaNi₅H are plotted and comparedwith each other in detail. Owing to the decrease of CD and the increase ofpotential barrier between La and Ni (2c) atoms, the hydrides LaNi₅H_x become lessstable gradually with the increase of hydrogen atoms.For LaNi₅H_5.5,5He_0.5,1, the optimized results show that helium atoms preferoccupying the tetrahedral interstitial sites (6c₁) to locating the 1b sites, which isconsistent with the analytical results of XRD. From the analysis of density ofstates and electron densities, with the increase of helium content, the valencebands are broader gradually. A strong and narrow peak appears in the low energyregion (about -4.0eV), which is due to the contribution of the He-s band. Inaddition, the charges of helium atom are well localized and the charge densitiesaround He nucleus is rather low, and no clear band charges between He and La, Niatoms can be seen, in other words, helium atom lies in the deep potential trap.The structures LaNi₅H_6.5,6He_0.5,1 are calculated in theory for the first time.The calculated data also indicate that helium atoms prefer to occupy the.tetrahedral interstitial sites (6c₁), which is agreement with the optimized results ofLaNi₅H_5.5,5He_0.5,1. Compared to the cell parameters of LaNi₅H₇, the cell parametera value increases about 0.05(?), on the contrary, the variation trend of a value becomes decrease gradually.To further study the behavior of heiium atoms in hydride LaNi_4.5Al_0.5H₆, thestable structure of LaNi_4.5Al_0.5H₆ is firstly obtained. Based on this, the behavior ofhelium atoms in the structures LaNi_4.5Al_0.5H_5.5He_0.5 and LaNi_4.5Al_0.5H₅He areelementarily researched.