First-Principles Study of the Hydrogen-Metal System

The ab initio pseudopotential method within the local-density-functional formalism with a plane-wave basis is implemented to investigate the various properties of hydrogen in yttrium. With a "soft" pseudopotential scheme, satisfactory convergence is achieved with an energy cutoff of 30-40 Ry. It is found that a correction in the exchange -correlation functional is needed to handle the outer core contribution in this early transition metal. The overall results for the structural properties of bulk yttrium are in good agreement with experiments. In the solid solution phase of hydrogen in hexagonal close-packed yttrium ( alpha-YH_{x}, x < 0.5), the occupation of the tetrahedral site is found to be energetically favorable. The calculated vibrational frequency in a one-dimensional double-well potential along the c axis and the diffusion barrier from one tetrahedral site to the neighboring octahedral site are in excellent agreement with experiments. It is found that the pairing of hydrogen across a metal atom is indeed energetically favorable, which is explained from the electronic structure of the system. The study of beta -YH_{2 + x} is carried out with the metal atoms forming a face-centered cubic lattice and with 0 <= x <= 1. The total energy, electronic and structural properties, density of states (DOS), angular-momentum-decomposed DOS, and charge transfers are investigated by employing the supercell modeling method. Interesting phenomena such as the volume contraction with increasing x and the (420) -plane ordering of hydrogen are studied. Finally, yttrium trihydride with the metal atoms in the hexagonal close -packed (HCP) structure is examined. The calculations confirm that hexagonal YH_3 with wave-like hydrogen displacements is energetically more stable than the cubic structure, and is consistent with the neutron diffraction data for YD_3 and HoD _3. These hydrogen displacements are found to be Peierls-like distortions and are associated with the charge density wave in this three-dimensional system. The calculated final LDA band structure for YH _3 yields a semimetal rather than a semiconductor. However, the possible existence of an excitonic insulating ground state is speculated based on the unusual electronic structure in this system.