Theoretical Study Of Structures And Properties Of M-H (M=Ti, V, Zr, Nb) Systems Under High Pressure

Hydrogen energy is one of the ideal solutions for solving the energy crisis and environmental pollution problems.The key of large-scale industrial applications of hydrogen energy lies in production,storage and utilization.In the storage process,traditional hydrogen storage methods are dangerous and inefficient.It is imperative to find new hydrogen storage methods.Most of the metals can react with hydrogen.Metal hydride has been widely concerned as hydrogen storage materials.Titanium,vanadium,zirconium and niobium are used as components of hydrogen storage alloys frequently,which are generally form dihydrides.In order to enhance the hydrogen storage capacity,the formation conditions of the higher hydrogen content hydrides have been highly interested.The formation conditions predicted by calculation can be a good guide for experimental synthesis.In chapter 2,we introduce the theoretical research methods used in this thesis.The first-principles calculation method based on density functional theory is described briefly,followed by the particle swarm optimization algorithm for structure searches.Finally,the computational details such as calculation parameters in this work have been discussed.In the structural search,we need to find the lowest energy structure in the each stoichiometry.Tetrahydride is the highest hydrogen content hydrides in our structural search.We compared the low-energy structures in each species and calculated the enthalpies curves of these structures under pressure of 0 to 100 GPa to evaluate the stability.The stability among different stoichiometry for hydrides was evaluated by formation enthalpy.The formation enthalpies of stable phases as a function of hydrogen chemical potential under high pressure were calcuated to obtain pressure-chemical potential?P-?_H?phase diagrams.The dynamical stability and electronic structures have also been investigated for the stable hydrogen content hydrides by the calculations of the phonon spectrums and projected electronic density of states.We demonstrate that the dihydrides are the highest hydrogen content compounds available under ambient pressure in these four metal-hydrogen systems.Stable structures of MH_2.5 can be formed in all the four systems under high pressure conditions.Meanwhile,V-H system cannot form stable trihydride below 100 GPa.Ti and Nb can form stable trihydride with H under the lowest pressure condition of 15 GPa and 33 GPa respectively.Zr is the only one that can form both tetrahydride and trihydride,and the corresponding pressure conditions are not exceed 5 GPa.After comparing with the experimental and theoretical results in the literature,ten novel stable phases,R3?-TiH₃,P4₂/mnm-TiH₃,Ibam-TiH_2.5,Cccm-VH,Pna2₁-ZrH₄,R3?-ZrH₃,Pm3?n₁-ZrH₃,R3?m-ZrH_2.5,P2/c-NbH_2.5 and P4₂/mcm-NbH_2.5 were discovered under various pressures in this work.In particular,the Pna2₁-ZrH₄ is found to be stable under pressure of 4 GPa with high hydrogen content.These finding of stability conditions for the high hydrogen content hydrides will provide guidance for their preparation in experiments.