Electronic Structure And Optical Properties Of Hydrogen-Doped Lithium System Under Pressure

This paper presents ab initio studies on the system of bulk lithium doped by hydrogen atom, for understanding the role how impurity and external pressure influence its structure and properties of metal lithium.At first, the stable structure of the doped system has been researched by ab initio molecular dynamics. The starting lattice is a body center cubic supercell which consists 16 lithium atoms with one substituted by a hydrogen atom. By ab initio dynamical iterations at different temperatures, we find that the hydrogen atom runs far away from its original site to an interstitial site with 6 lithium neighbors, leaving a defect of empty site. It can be concluded that a hydrogen atom is an interstitial impurity in bcc lithium rather than substitutional. Secondly, for obtaining the exact lattice structure of the doped system, geometry optimization at 0K have been performed, from a starting-up lattice of 16-atom bcc lithium with an interstitial hydrogen atom impurity. The result shows its equilibrate structure belongs to D4h symmetry. When external pressures is applied, the ratio of the doped lattice parameters a/c changes irregularly. Then the electronic structure and dielectric response of the doped system are studied at 0 K. and several pressures.Finally, based on the optimization crystal structures, the electronic structure and dielectric response are then calculated at different pressures, using pseudopotential-plane-wave method within the framework of density functional theory. By all the calculations, we draw conclusions as follows: 1) With the hydrogen atom doped in, the electronic structure is dramatically changed. An isolated band with a band gap is generated below Fermi level. The lower valence bands are pulled down, while the upper ones pushed upward. The same phenomena occurs to the conduction bands. The bands are distorted due to the different potentials of Li and H. the most important parallel bands along certain axes contribute to the infrared reflecttivity. 2) With the pressure up to 2~4GPa, extra band distortion appears. The higher of the pressure, the more serious distorted of the bands. When the pressure is great or equals to 4eV, an additional reflectance dip emerges at the frequency of 4.3eV. It can be concluded that the hydrogen atom acts as a negative ion in the system, at least an ionic center. It attracts some free electrons closer,。 Other electrons may feel a repulsive potential and their energies may be pushed up. All these cause the dielectric response and the optical properties to be different.