Properties Of Hydrogen Diffusion In Transition Metal Palladium Based On First-Principles

With the rapid development of the world economy, more and more importance has been attached to energy exploration and environmental protection. Hydrogen energy has become the most promising clean energy, energy research, especially theoretical studies aiming at improving the hydrogen storage properties of hydride has been considered more and more important. It can be used as energy storage materials. Some new properties of metals may emerge when hydrogen atoms are added to the metals, such as the metal magnetic properties, or superconductivity. Therefore, the study on metal-hydrogen system has been one of the most influential and significant research fields in recent years, and research has shown that practical application values exist in metal-hydrogen.Density functional theory has been an important tool in the electronic properties on the condensed matter systems. In recent decades, the study on hydrogen in metals or other solids with the first principles based on density functional theory has been widely put into application. In this paper, the diffusion behavior of hydrogen atoms in face-centred cubic transition metal palladium unit cell is studied with the first principles method based on density functional theory.First, the embedding energy of one hydrogen atom in interstitial sites of the face-centered cubic transition metal palladium and the activation energy of hydrogen diffusion along different paths is studied. We find that the octahedral interstitial sites are the most stable sites for a single hydrogen atom in Pd, and the tetrahedral sites are the second most stable sits, with respective embedding energy values of-2.4346eV and-2.1283eV. By comparing the activation energy of hydrogen diffusion along different paths, we find that the most favorable pathway for H diffusion in face-centered cubic of Pd is the indirect O-T-0 pathway.Second, by studying the magnetic moments and electronic structures of individual hydrogen atoms diffusing in face-centered cubic structure of transition metal Pd, we find that the results of magnetic moments are exactly the same in the two direct octahedral interstitial site-octahedral interstitial site (O-O) diffusion paths—i.e. the magnetic moments are the largest in the octahedral interstitial sites, and the magnetic moments are the lowest in saddle point positions. The same conclusion is drawn from the study of indirect diffusion path for magnetic moments, with the exception that there are two saddle points, distributing symmetrically between in the octahedral interstitial sites and tetrahedral interstitial sites, with the magnetic moments in tetrahedral interstitial sites being the local maximum points. Magnetic moments are decided by the distance between hydrogen and palladium atoms. We also study on the density of states of some special points, with the result that the density of states near the Fermi level is mainly contributed by 4d electrons of Pd, and there exists distinctive density of states distribution with the energy ranging from -10.0eV to-6.0eV, contributed by ls electrons of H. The density of state values is the largest in the saddle points and tetrahedron interstitial sites in the O-T-O path, which is the same with the result of O-T-O diffusion path being the optimal path.Finally, we study the change of magnetic moments with the change of cell volume in unit cell of transition metal Pd with one embedded hydrogen atom. Results show that there is a "magneto-volume effect" when the hydrogen atom is in octahedral interstitial sites of transition metal Pd, namely, there exists the rule of magneto-volume effect from "low magnetic moment-low cell volume" to "high magnetic moment - large cell volume" with the change of the cell volume. The magnetic moments have not obviously changed with the cell volume when the hydrogen atom is in tetrahedral interstitial sites of transition metal Pd, which displays the same result with that in transition metal Pd. At the same time, the lower the nearest neighbor Pd-H distances are, the lower the magnetic moments are, the larger the nearest neighbor Pd-H distances are, the larger the magnetic moments are.