Surface-phonon instabilities of W(001)

Tight-binding calculations of the surface-phonon dispersion curves for unreconstructed p(1×{}1) W(001) reveal a whole region of soft surface modes at the boundary of the surface Brillouin zone, caused by interactions with electronic surface states around EF. An interplay of Fermi-surface nesting features and real-space d-orbital interactions leads to the maximum instability of the M₅ phonons, the basis of the observed c(2×{}2) reconstruction patterns. For small H coverages the electronic surface states are not removed but modified, probably in such a way as to produce incommensurate reconstructions.