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 ${\mathrm{cm}}^{\mathrm{-}1}$) in the frequency of the ${\mathrm{W}}_2$H symmetric stretch mode, ${\nu }_1$, is observed at low coverages (CTHETA≤0.3), indicating a local structural rearrangement (≊0.03 Å) 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 (${\nu }_1$≊1300 ${\mathrm{cm}}^{\mathrm{-}1}$) separated by domain walls of finite size and with unreconstructed W atoms as evidenced by a new H vibration at ≊1100 ${\mathrm{cm}}^{\mathrm{-}1}$. 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).