High-resolution helium-atom-scattering measurements of angular distributions and time-of-flight spectra are reported for W(001) in the temperature range from 200 to 1900 K. Angular variations in the total intensity show a reconstruction related superstructure peak whose angular position and intensity are temperature dependent in the range from 400 to 220 K. At 220 K the peak intensity is maximum and its position corresponds to that expected for a reconstructed (√2 × √2 ) R45° low-temperature phase. Time-of-flight spectra show that the behavior of the superstructure peak at higher temperatures is attributable to the elastic component and must therefore be due to a surface structural feature. Of the measured phonon inelastic peaks in the time-of-flight spectra, the one assigned to a longitudinal mode shows the typical strong temperature dependence of a soft mode in the temperature range 450≥T≥220 K. For T≤220 K two new phonon modes appear, associated with the Brillouin-zone boundary corresponding to the c(2×2) phase. Both observations suggest that the overall phase transition on W(001) is of the ‘‘displacive’’ and not the ‘‘order-disorder’’ type, as inferred from several recent structural studies. All experimental findings are consistent with a charge-density-wave mechanism driving the reconstruction in agreement with the recent theory of Wang and Weber [Phys. Rev. Lett. 58, 1452 (1987)].

• Received 22 June 1992

DOI:https://doi.org/10.1103/PhysRevB.46.16081