HELIUM-ATOM-SCATTERING STUDY OF THE STRUCTURE AND PHONON DYNAMICS OF THE W(001) SURFACE BETWEEN 200-K AND 1900-K

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 (square-root 2 X square-root 2)R45-degrees 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 greater-than-or-equal-to T greater-than-or-equal-to 220 K. For T less-than-or-equal-to 220 K two new phonon modes appear, associated with the Brillouin-zone boundary corresponding to the c(2X2) 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)].