Structure and energetics of half-monolayer hydrogen on W(110)

We have studied the hydrogen ordering on the W(110) surface by using the first-principles pseudopotential approach based on the local density approximation. The calculations for a half monolayer coverage of hydrogen indicate that the three-fold hollow site is favored for this coverage, similar to the full-coverage case. A p(1 x 2) ordering is found to have the lowest energy, in agreement with low-energy electron diffraction(LEED) results. However, another type of ordering, p(2 x 1), is also close in energy. The possibility of a phase coexistence and a subsequent change in the diffraction pattern are discussed. We find small displacements that are in opposite directions for two inequivalent surface W atoms in one unit cell. Without a uniform lateral shift of surface atoms, the LEED intensities calculated using the theoretical structure parameters still exhibit an asymmetry in integer spots at this coverage, resulting from the ordering of hollow-site hydrogen.