  Understanding the behavior of hydrogen in hexagonal tungsten carbide (WC) is
of particular interest for fusion reactor design due to the presence of WC in
the divertor of fusion reactors. Therefore, we use first-principles
calculations to study the hydrogen behavior in WC. The most stable interstitial
site for the hydrogen atom is the projection of the octahedral interstitial
site on tungsten basal plane, followed by the site near the projection of the
octahedral interstitial site on carbon basal plane. The binding energy between
two interstitial hydrogen atoms is negative, suggesting that hydrogen itself is
not capable of trapping other hydrogen atoms to form a hydrogen molecule. The
calculated results on the interaction between hydrogen and vacancy indicate
that the hydrogen atom is energetically trapped by vacancy and the hydrogen
molecule can not be formed in mono-vacancy. In addition, the hydrogen atom
bound to carbon is only found in tungsten vacancy. We also study the migrations
of hydrogen in WC and find that the interstitial hydrogen atom prefers to
diffusion along the c axis. Our studies on the hydrogen behavior in WC provide
some explanations for the experimental results of the thermal desorption
process of energetic hydrogen ion implanted into WC.
