| Summary: | 博士 === 國立臺灣大學 === 物理學研究所 === 92 === In this thesis, we used full-potential linearized augmented-plane-wave method (FLAPW), and generalized gradient approximation (GGA) based on first-principles density-functional theory (DFT), to study the electronic structure and relative physical properties of a series of transition metals. The results are compared and discussed to the experimental values and other theories.
In the first part, we calculated lattice constants, bulk modulii, X-ray absorption near-edge spectra (XANES) and optical properties of Ni3Al, Ni3Ga and Ni3In, which are in good agreement with experiments. It is predicted that all three compounds have similar band structures and weak ferromagnetic. The measured Ni K-edge XANES are well explained by the theoretical XANES and unoccupied Ni-p density of states (DOS), suggesting that XANES is a useful probe of the electronic structure of the intermetallics. However, due to a small difference between theory and experimental Ni K-edge XANES spectra in Ni3In, there are still issues in its structure and magnetism. Therefore, we have carried out three similar structure calculations for Ni3In in the cubic L12, tetragonal D022 and hexagonal D019 structures. It is predicted that Ni3In would be a weak ferromagnet in the L12 structure at low temperatures and would become a paramagnet with the D022 structure as the temperature is increased to room temperature or above. It is also predicted that under high pressures of 128 kbars, Ni3In would undergo the same phase transition at low temperatures. This is different to Ni3Al and other relative compounds .We hope that these interesting theoretical findings would stimulate further experimental investigations such as temperature-dependent structural, specific-heat, and magnetization experiments on this nearly or weakly magnetic intermetallic compound.
For another series of materials, such as Fe3Al, Fe2VAl and Fe2VGa the theoretical studies of crystal structures, bulk modulii, XANES spectra and magnetic properties are in agreement with experiments. Both theories and experiments have proved that Fe2VAl and Fe2VGa (both are L21 structure) are nonmagnetic semi-metal. On the other hand, the Fe3Al (D03 structure) is a ferromagnetic metal. Therefore the magnetism has huge dependence on its structures. Because the theory of XANES spectra is almost consistent to the experiments, and the experimental XANES features for these intermetallic compounds reflect the Fe- and V-p unoccupied partial DOS. This proves that the precision of experimental measurement, and using FLAPW method and GGA to study these materials are reliable.
Finally, we simulate CeCo2 nano-particle for a supercell slab with a 14.32 Å thickness, and using GGA and GGA+U method to perform the calculation. Qualitatively, both results and measurement of nano-particle all possess magnetism. However, from theories and experiments, the bulk CeCo2 does not possess magnetism. Therefore, this suggests that surface effect may play a significant role in the magnetism observed in the nano-particle.
In summary, in this thesis we use first-principle to perform calculations on material properties of above transition metals. The results are in good agreement with experiments. We hope that these detailed calculations on band structures are useful for clearer, deeper understanding to microscopic properties of these materials. We also hope that these materials will have contributions to the development of technologically in the future.
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