Abstract
We investigate the structure and energetics of interstitial hydrogen and hydrogen molecules in layered , an issue of interest both for hydrogen storage applications and for the use of as an (opto)electronic material. Using first-principles density functional theory we find that hydrogen interstitials are deep donors. molecules are electrically inactive and energetically more stable than hydrogen interstitials. Their equilibrium position is the hollow site of the layers. The migration barrier of a hydrogen molecule is calculated to be smaller than 0.6 eV. We have also explored the insertion energies of hydrogen molecules as a function of hydrogen concentration in . For low concentrations, additional inserted molecules prefer to be located in hollow sites (on top of the center of a hexagon) in the vicinity of an occupied site. Once two molecules have been inserted, the energy cost for inserting additional molecules becomes much lower. Once all hollow sites are filled, the energy cost increases, but only by a modest amount. We find that up to molecules can be accommodated within the same interlayer spacing of an areal supercell.
- Received 31 March 2016
- Revised 25 July 2016
DOI:https://doi.org/10.1103/PhysRevB.94.085426
©2016 American Physical Society