| 其他摘要 | With the recent advances in algorithms and computer hardwares, first-principles method based on the density-functional theory (DFT) has become a powerful tool for investigating properties of materials. In this dissertation, using discrete variational Xα (DV-Xα), plane-wave ultrasoft pseudopotential (PW-USPP), and other first-principles methods we mainly focus on the investigations on some fundamental problems of hydrogen storage materials, like helium retention in tritides, isotopic effects, structural transition, and predicting crystal structure. The main conclusions are obtained as following:
1.The alloying effect on the electronic structure of LaNi4M tritides is investigated using the first-principles discrete variational Xα (DV-Xα) method. The calculated results show that the covalent interaction between atoms will play a much more important role in studying the alloying effect on chemical bonding characteristics in LaNi4M tritides than ionic interaction. It is also found that in LaNi4M tritides helium forms stronger covalent bonds with the weaker hydride forming elements than La. By analyzing the relation between the binding energy difference and bond order, our study indicates that after some alloying elements substituting for Ni locating in 3g site in tritides, the helium retention capability becomes stronger and changes as the following sequence: Al > Cr > Mn > Fe > Co > Ni, and is also very distinct for Cu although the chemical bonding between Cu atom and Ni atom is degraded drastically.
2.A series of investigations of mechanical and thermal properties of titanium hydrides, deuterides and tritides have been performed, however, very limited theoretical studies of thermodynamic properties for them can be found. Based on density-functional theory (DFT) and density-functional perturbation theory (DFPT) we have discussed systematically the hydrogen isotope effects on the thermodynamic properties of TiX2 (X=H, D, and T) system. Our calculations indicate that for evaluating accurately their physical properties at absolute zero temperature, such as the equilibrium lattice constants, bulk modulus, and heat of formation, the zero-point energy correction must be taken into account. By performing the phonon calculation within quasiharmonic approximation (QHA), we obtain their vibrational free energies, vibrational entropies, and temperature dependence of specific heat, thermal expansion, and bulk modulus. Those results demonstrate that compared to TiH2, TiT2 and TiD2 are more stable and the zero-point effects play an important role in their thermal expansion. The increase in the force constant between Ti and H causes the higher value of specific heat of TiH2 during the phase transition from FCC to FCT. In addition, comparing with available experimental values, we can conclude that QHA is feasible for describing the thermal properties of TiX2.
3.We present a systematic first-principles investigation on the high-pressure structural stability of Mg(AlH4)2 using a plane-wave pseudo-potential method. The total-energy calculations show that at ambient pressure the structure of α-Mg(AlH4)2 determined by experiments is more stable than the other proposed structures and with increasing pressure the α to β (δ-Zr(MoO4)2-type structure) and β to γ (Ca(BF4)2-type structure) transitions occur at 0.67 and 10.28 GPa accompanied by the volume reductions of 6.6% and 8.7%, respectively. A detailed study of the electronic structure reveals the covalent bonding characteristics between Al and H in AlH4 subunit, ionic bonding interaction between Mg and AlH4, and the nonmetallic features of α, β, and γ phases even at pressures up to 20.0 GPa. The changes in electronic structures are mainly responsible for the relative stability of the three phases under high pressures. Finally, according to the analysis of structural relations between them we believe it is capable of producing α→β→γ structural transitions at high pressures.
4.A systematic theoretical investigation on the high-pressure structural stability of Li2BeH4 has been performed by using first-principles method based on DFT. Total energy calculations show that at ambient pressure the structure of α-Li2BeH4 observed in experiments is more stable than the other proposed structures in this work and the structural transformation from α to β (Cs2MgH4-type; Pnma) occurs at 18.1 GPa together with a volume reduction of 4.7%. A detailed study of their electronic structures under ambient pressure to 30.0 GPa reveals that this behavior is closely related to the variation in the Be-H covalent bonding in BeH4 anionic subunits of Li2BeH4. Based on a colligated analysis of the covalent bond number per unit area (Na) and the scaled bond overlap population (BOPs), β-NaAlH4 and β-Mg(AlH4)2 are expected to be the promising candidates for hydrogen storage among the other investigated materials. However, the improvement of hydrogen absorption/desorption for Li2BeH4 is less significant.
5.Using systematic first principles total energy calculations based on DFT, the crystal structure of Na2BeH4 was first predicted. Its crystal structure at ambient conditions can be characterized by α-K2ZnBr4-type monoclinic structure (space group: P21/c). With increasing pressure the α to β (α-Cs2MgH4-type; space group: Pnma) structural transition occurs at 1.1 GPa accompanied by a volume reductions of 8.7%. The density of states (DOS) and crystal overlap hamiltonian population (COHP) analysis show that the distinct covalent-bonding interaction prevails in BeH4 subunits and the whole crystal exhibits the nonmetallic features. The relative thermal stability between α and β is further investigated by performing phonon calculations based on DFPT and QHA. The calculated results like free energies, vibrational entropies show that with increasing temperature, α-Na2BeH4 is always more stable than β-Na2BeH4 and the possibility of the occurrence of α→β structural transition is tiny. |
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