Crystal Structure Prediction And High-pressure Behavior Of Light Complex Hydrides:a First-principle Study

Complex hydrides composed of light weight elements such as amides/imides, borohydrides and alanates have attracted great attention as one of the most promising materials for hydrogen storage due to their high theoretical gravimetric density and low price. Base on the previous experimental or theoretical results, we have investigated systemically and deeply the high-pressure crystal structure, electronic structure, and vibrational properties of some light complex hydrides using first-principles calculations and the ab-initio evolutionary structure prediction simulations. The main conclusions obtained from our theoretical calculations are given as below:(1) A detailed study of the high-pressure structural stability in LiNH2is performed. Two high-pressure polymorphs,β-LiNH2(orthorhombic, Fddd) and;γ-LiNH2(orthorhombic, P21212), are found to be more stable than the ground-state a-LiNH2(orthorhombic,1-4) with increasing pressure. The βâ†'γstructural transition occurs at10.7GPa, which is in good agreement with experimental observation. Further analysis of the structural properties, charge density distribution, and calculated phonon density of states for all the competing phases shows the N-H…N hydrogen bond obviously emerges in the high-pressure γ-LiNH2. The existence of hydrogen bond elongates the N-H bond length and weakens the N-H polar covalent bonds within NH2groups, which may be beneficial to the hydrogen release from γ-LiNH2.(2) The investigations on the high-pressure behavior of NaNH2show that NaNH2undergoes two pressure-induced structural transitions in the pressure range from0to20GPa. The ground-state a-NaNH2first transforms into an orthorhombic β-NaNH2with space group P21212at2.2GPa and then into a monoclinic y-NaNH2with space group C2/c at9.4GPa, accompanied by the volume reductions of11.3%and1.2%, respectively. The predicted transition sequence(Fdddâ†'P21212â†'C2/c) of NaNH2is very similar to the high-pressure behavior of its counterpart LiNH2(I-4â†'Fdddâ†'P21212), suggesting that the trend of lowing symmetry induced by high-pressure may be common in alkali metal amides. Pressure not only leads to an increase in the orientational order of the [NH2]-amides anions in NaNH2but also simultaneously shortens the intermolecular N-H distances between the two neighboring [NH2]-amides anions, which yields the emergence of the N-H…N hydrogen bond in β-and γ-NaNH2. Our calculated results also show that the strength of the approximately linear N-H… bond in β-NaNH2is expected to be stronger than that of the bent N-H…N bond in γ-NaNH2.(3) The high-pressure crystal structure, electronic structure, and lattice dynamical properties of Li2NH are investigated systematically. The total-energy calculations show that the Pbcaâ†'Cmcm and Cmcmâ†'P63mc structural transitions occur respectively at3.2and14.8GPa and the obvious pressure-induced ordering appears under compressing Li2NH. A detailed study on the density of states and electron localization function reveals that all the three stable structures of Li2NH are nonmetallic with the typical ionic bonding between Li+cations and [NH]2-imide anions and the strong N-H covalent bonding prevails in each [NH]2-unit. No imaginary phonon frequencies are found in the ground-state Pbca phase, indicating this structure is dynamically stable. The calculated phonon density of states for the Pbca phase consists of two frequencies bands:the low-frequency band below844cm-1being ascribed to the motion of the Li, N and H atoms and the high-pressure band above3172cm-1resulting from the N-H bond stretching.(4) The latest ground-state structure of LiBH4(space group:Pnma; lattice parameters:a=8.564A, b=4.352A, c=5.753A) reported by Tekin et al. has been reproduced by our evolutionary structure prediction simulations. However, our total-energy calculations show that although the Pnma structure predicted by Tekin et al. exhibits a significant structural difference in comparison with the experimental Pnma structure determined by Soulie et al., the difference in their calculated total energies is only about1meV/f.u.. The ground-state structure of LiBH4is insensitive to the variation of energy, which may be one of the important reasons why the structural data of the ground-state phase of LiBH4still remains controversial in experimental or theoretical studies. The ground-state orthorhombic Pnma structure transforms into the tetragonal P-421c structure with increasing pressure to10GPa, which is in good agreement with the recent experiment. The further analysis on electronic structure shows that the ground-state Pnma and high-pressure P-421c structures belong to the typical ionic compounds with a large band gap (>5eV) and the strong polarized covalent exists between B and H atoms in the tetrahedal [BH4]-units.