  We examine the behaviour of hydrogen ions, atoms and molecules in alpha-boron
using density functional calculations. Hydrogen behaves as a negative-U centre,
with positive H ions preferring to sit off-center on inter-layer bonds and
negative H ions sitting preferably at in-plane sites between three B12
icosahedra. Hydrogen atoms inside B12 icosahedral cages are unstable, drifting
off-center and leaving the cage with only a 0.09 eV barrier. While H0 is
extremely mobile (diffusion barrier 0.25 eV), H+ and H- have higher diffusion
barriers of 0.9 eV. Once mobile these defects will combine, forming H2 in the
interstitial void space, which will remain trapped in the lattice until high
temperatures. Based on these results we discuss potential differences for
hydrogen behaviour in beta-boron, and compare with experimental
muon-implantation data.
