Theoretical Study Of Structural Stability And Hydrogen Storage Performance For Laves Phase Alloy

Because of the great potential in energy storage materials and high temperature materials,the AB₂ Laves phase alloys have been widely concerned by theoretical and experimental researchers.Through the substitution and doping of foreign atoms,multi-component alloys are constructed by replacing solid solution based on the unique lattice structure,an important approach to modulate the structural properties and improve the properties of alloy materials.The detailed study of the physical and chemical properties of the new structure by theoretical calculation not only help to understand the atomic interaction in the system,but also provide a performance prediction for the expected synthesis and preparation of materials.Based on the first-principles calculation and statistical ensemble theory,the structural stability of YFe₂and La Fe₂binary alloys and V_2xFe_2（1-x）Zr/Sc_xY_1-xFe₂（0≤x≤1）quasi binary alloys are studied in detail.The hydrogen storage properties of the target alloys and hydrogenated phases are explored.The results will be helpful for the hydrogen storage properties of the target alloys and hydrogenated phases.It provides a theoretical basis for the design and development of high-performance alloys and hydrogen storage materials.For the Fe_xY_1-x and Fe_xLa_1-x（0<x<1）binary alloys,it is confirmed that YFe₂and La Fe₂are stable structures according to the formation enthalpy convex analysis.The fitting of thermodynamic function and equation of state shows that YFe₂has higher thermal stability than La Fe₂,while La Fe₂ system is prone to instability at high temperature.Phonon spectra also confirms the dynamic stability of YFe₂and La Fe₂,with the vibration intensity of YFe₂higher than that of La Fe₂.The highest binding energy of YFe₂and La Fe₂ hydrogenated phase is-0.206 e V/H at 5 H/f.u..The hydrogen content corresponding to the lowest enthalpies of formation in YFe₂H_x and La Fe₂H_x are 4.5 H/f.u.and 4 H/f.u.,respectively,indicating that the hydrogen storage capacity of YFe₂ alloy is higher than that of La Fe₂.When the hydrogen content is lower than 1 H/f.u.,the H atom tends to enter the B₄ site,while at higher hydrogen content,the H atom tends to enter the A₂B₂ site.In a certain range,the increase of hydrogen content leads to an increasement of mechanical strength of hydrogen storage system,and a degration with the increase of temperature.Compared with La Fe₂,YFe₂ hydrogenated phase maintains a large degree of elastic strength and elastic isotropy,showing better hydrogen storage performance.In order to improve the hydrogen storage properties of the alloys,we have studied the structural stability of V_2xFe_2（1-x）Zr、Sc_xY_1-xFe₂（0≤x≤1）quasi binary alloys.It is found that V_2xFe_2（1-x）Zr alloy can obtain ordered stable structures of different components at 0 K,while the single-phase solid solution structure of Y_1-xSc_xFe₂ system only stabilizes at finite temperature,in line with the corresponding experimental preparation conditions.Considering the contribution of configuration entropy,the calculation shows that the highest critical solution temperature of Y_1-xSc_xFe₂ system is around 600 K,while the highest critical temperature of order disorder transition of V_2xFe_2（1-x）Zr system is around 155 K.The total energies of all non equivalent structures in V_2xFe_2（1-x）Zr and Y_1-xSc_xFe₂ alloys are obtained by first principles calculation,and explained by a simple bond energy model.Comparing with the nearest neighbor interaction coefficient,it can be judged that the V-Zr interaction is the strongest in V_2xFe_2（1-x）Zr alloy,and the substitution of V atom for Fe atom is beneficial to improve the structural stability of the ternary alloy.The electronic properties show that the V_2xFe_2（1-x）Zr alloy system mainly consists of metal bonds and partially ionic bonds.The maximum bond strength between V-Zr atoms is observed,which is consistent with the interaction energy in the bond energy model.The relation between mechanical properties and performance of hydrogen storage materials is also discussed,as excellent hydrogen storage materials requires good elastic property to keep their stability during de/hydrogenation process.The ratio of shear modulus to bulk modulus is less than 0.57,which indicates that the alloy system has certain plasticity.The elastic anisotropy factor and three-dimensional equipotential surface show that the substitution of V atom for Fe atom in V_2xFe_2（1-x）Zr system can reduce its elastic anisotropy,and the component structure of VFe Zr shows the highest degree of elastic isotropy.With the help of bond energy model,the elastic strength of V_2xFe_2（1-x）Zr alloy can also be reasonably predicted,and the interaction coefficient shows that the interaction between Fe and Zr atoms in the structure has the greatest contribution to the elastic strength of the system.The addition of Sc element in Sc_xY_1-xFe₂ system can enhance the elastic strength of the system,and the increase of temperature will lead to a decrease of the elastic strength of the alloy system.The maximum hydrogen storage content of Sc_xY_1-xFe₂ and V_2xFe_2（1-x）Zr ternary alloys can reach respectively 4.29%and 3.88%（8 H/f.u.）,which is higher than 2.19%（4.5 H/f.u.）of YFe₂binary alloy.The hydrogen storage capacity of the alloy can be improved by adjusting and increasing the number of components,which provides a theoretical basis for the improvement of the properties of hydrogen storage alloys and the design and development of multi-component hydrogen storage alloys.