| Energy is the foundation of subsistence and development for modern society.It becomes the global consensus that hydrogen energy substitutes fossil fuels due to hydrogen's high combustion value,no pollution and wealthy resource.The hydrogen storage is the most difficult technological problem in the development of this novel energy.In recent years,M-N-H(M:element of I-IV group or some transition metal) metal complex systems as hydrogen storage material have attracted much attention for its high weight density of hydrogen storage and quality of mutually inverse.In this work,the structure and properties of the M-N-H(M=Li,Na,Mg,Ca) clusters are investigated by using first principle calculations.In chapter 2,we optimized the structures of the(Li3N)n(n=1～5),(Na3N)n(n=1～4) and (Ca3N2)n(n=1～4) clusters,then the properties of the ground state of these clusters were calculated.The calculated results show that,for(Li3N)n(n=1-5) and(Na3N)n(n=1-4) clusters,the N atom occupies the center of every structure unit of clusters and prefer to 4 or 5 coordination number,and the most stable structures are favorable with 5 coordination number of the N.While the(Mg3N2)n(n=1～4) and(Ca3N2)n(n=1～4) clusters tend to be cage geometry with the bonds composed of the N and metal atom,and the geometry of the most stable(Mg3N2)2 clusters with the higher symmetry(Td) has higher dynamical stability,and this structural unit always exists in bigger(Mg3N2)n clusters. This character indicates some special importance for the clusters.The charge transferring occurs between metal atoms(i.e.Li,Na,Mg,Ca,etc.) and the N during them forming clusters.The charge transfer causes the positive of metallic atoms and the negative of N atoms,the bonds between the metallic atoms and the N is strong ionicity.In chapter 3,we optimized the structures of the ground state for(LiNH2)n(n=1～5), (NaNH2)n(n=1～5),[Mg(NH2)2]n(n=1～5) and[Ca(NH2)2]n(n=1～5) clusters,and then got the properties of these clusters.The calculated results show that(LiNH2)n,(NaNH2)n,and [Ca(NH2)2]n clusters incline to form ring-like structures,while the[Mg(NH2)2]n clusters tend to be chain-like structure.The bond lengths of clusters generally agree with that of crystals.The IR and Raman spectra of these clusters are distributed three region,the vibration near about 1600cm-1 are related to 2n bond bending mode of weak H-N-H bonds, but the vibration near about 3350～3620cm-1 are ascribed to stretch mode of N-H bonds (including one symmetry and one anti-symmetry mode of N-H stretch),these two region are all the characteristic spectra of -NH2 group.The geometries and the vibration spectra of these clusters suggest that -NH2 groups do almost not change its structure both in clusters and in crystals,and therefore they keep the integrated character of -NH2 groups. Thus,hydrogen totally store in -NH2 groups,which indicate that the clusters can give a good exploration for the hydrogen storage properties of the crystal.The metallic atoms and H atoms in the above mentioned clusters carry positive charge and the N atoms carry negative charge.Therefore,the interaction between the metal atoms and the -NH2 groups shows strong ionic character and the N-H bonds are covalent.In chapter 4,the structure and properties of the ground states for(LiH)n(n=1～12), (NaH)n(n=1～12) and(MgH2)n(n=1～8) clusters are analyzed.The results suggest that the metallic atoms coordinated with 3-5 H atoms.The clusters tend to form six-membered or four-membered ring-like structures alternatively between the metallic atom and the H atom,especially double-layered six-membered ring-like and cubic structures are main structure unit to compose bigger clusters,and this is similar to that of crystal structures. The structure of(MgH2)n(n=1～8) clusters is composed of Mg-H bonds and Mg-Mg bonds, and the dominant coordination number is 4 and 3 for Mg and H,respectively.A smaller clusters of(MgH2)n(n=1～3) easily form a chain-like structures that were composed of four-membered ring-like unit alternatively between the metallic atora and the H atom. The most stable structures of tetramer and pentamer are mainly constructed by concave frames including 3 Mg atoms and 4 H atoms.The bigger cluster of(MgH2)n(n=6～8) easily constructed by the above mentioned concave frames and some kinds of CH4 structure(1 Mg and 4 H).The smaller clusters,such as MgH2,(MgH2)2 and(MgH2)3,have higher dynamical stability,and the chemistry activity of clusters were enhanced with the increasing of the cluster size.The charge transferring occurs between the metallic atoms and the N atoms during forming Li-H,N-H or Mg-H clusters.This transfer causes the metallic atoms positive and the H atoms negative,and strong ionic interaction between metallic atoms and H atoms.In chapter 5,we investigate the structures and properties of the ground states for (Li2NH)n(n=1～5) and[Li2Mg(NH)2]n(n=1～4) clusters.The results show that(Li2NH)n clusters tend to form a ring-like structures,and[Li2Mg(NH)2]n tend to form cage-like structures.Every N atom in the clusters is bonded to an H atom,and then they construct a -NH group.Hydrogen was stored totally in -NH groups.The bands of IR and Raman spectra dominantly occur at about 23.30～885.37cm-1 and 3326.93 cm-1～3547.02 cm-1. The latter band is totally due to the stretch vibrational mode of N-H bonds,and this is the characteristic spectra of -NH group.The interaction between Li,Mg and -NH group shows strong ionic character and the N-H in a -NH groups show covalent character.The -NH group and the -NH2 group are both covalent bonds.The former group is less stable than the latter,thus,the ability of dehydrogenation of the -NH group is stronger than that of the -NH2 group.The analysis of energy gap,Fermi energy and ionization potential of the above mentioned clusters suggest that Na nitrides,Na hydrides and Na amides have smaller ionization potentials and energy gaps,and higher Fermi energies.This indicates that Na-N-H systems have the most chemical activity and the best ability of hydrogen storage. On the basis of enthalpy of chemical reaction for the clusters,the inverse of dehydrogenating reaction for Mg-N-H systems are better,for the reaction of hydrogen storage and dehydrogenating,substitute MgH2 for LiH,or substitute Mg(NH2)2 for LiNH2 could decrease the absolute value of enthalpy of reaction,it indicates that the substitute may favor the reaction. |
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