Structural Stability And Hydrogen Storage Property Of YFe₂H_x?x=0?5?:A Theoretical Study

Laves phase intermetallic compound YFe₂ has considerable hydrogen storage capacity and high hydrogen-absorption/desorption kinetics,allowing it to be a competitive hydrogen storage material.However,in the process of hydrogen storage,the structure of YFe2 could be distorted easily,from the original cubic phase to hexagonal,orthorhombic,monoclinic,or many other phases,depending on the preparation condition and the hydrogen storage amount.This instability of the structure greatly limits the application of YFe₂ as hydrogen storage material.In this work,we focus on the structural stability of YFe₂ and its hydride YFe₂H_x,aiming to improve their stability for hydrogen storage.Using first-principle calculation,we have studied the stability of YFe₂H_x within cubic structure.Experimentists observed that A₂B₂ is always the most stable interstitial site as the lowest H concentration of x=1.2 was found for YFe₂H_x.Our calculations indicate that B4 is the most stable site before hydrogen concentration x is lower than 0.5.In line with experiment,A₂B₂ turns to be the most stable one beyond x=2.We ascribe the stability of B₄ site to the charge transfer and the structural difference,as the accumulated electrons around hydrogen atom from the host metal atoms would affect the stability of the interstitial sites.YFe₂ is a typical ferromagnetic Laves phase,and shows a decreasing magnetization along with the insertion of hydrogen.The amount of reduced magnetization depends on the types of site where the hydrogen atoms are inserted,and has a clear impact on the volume of the systems as well as their relative stability.There are 17 interstitial sites for the insertion of hydrogen atoms for per YFe₂ formula.By comparing the formation enthalpy of YFe₂H_x along with the chemical potential of hydrogen,we find that the cubic YFe₂H_0.5 and the orthorhombic YFe₂H₅ are stable thermodynamically.The relative stability of cubic and orthorhombic phases of YFe₂H_x is then investigated.The stability transition from cubic to orthorhombic phases is found to take place at x=1.5,a relative small value compare to the hydrogen capacity of YFe₂.In order to change the location of the transition point to maintain the cubic structure of the system,we adopt the external strain as well as the partial substitution of Y atom.It turns out that the tensile strains could maintain its cubic phase to a higher hydrogen concentration.Meanwhile,the substitution method reveals that the transition is mainly due to the chemical bonding rather than the size effect of the substitution metals.