H-induced structural phase transitions on W(100) by surface infrared spectroscopy

A vibrational study of the various H-induced phases on W(100) at room temperature has been carried out using surface infrared spectroscopy. A large shift ({}35 cm^-1) in the frequency of the W₂H symmetric stretch mode, ν₁, is observed at low coverages (CTHETA{≤}0.3), indicating a local structural rearrangement ({}0.03 A{}) of the W atoms as the commensurate ({}2 ×{} {}2 )R45^∘{} phase is established at CTHETA=0.3. The incommensurate phase, I-({}2 ×{} {}2 ), is made up of reconstructed domains (ν₁{}1300 cm^-1) separated by domain walls of finite size and with unreconstructed W atoms as evidenced by a new H vibration at {}1100 cm^-1. As the coverage is increased (CTHETA>0.5), the reconstructed domains decrease in size with a substantial expansion of the W dimer and finally disappear (CTHETA{}1). The disordered surface that ensues is brought back into registry with the bulk at saturation (CTHETA=2).