He-atom-beam scattering has been used to investigate the reconstruction of the W(001) surface. At low surface temperatures Tₛ a sharp (√2×√2)R45^∘ diffraction pattern is observed. When Tₛ is raised, the fractional-order beams broaden and shift towards the specular. At Tₛ≈450 K the time-of-flight spectra reveal two different surface phonons along the $〈110〉$ direction, a normal Rayleigh mode and a strongly anomalous mode with a maximum at a wave vector Q at one-half of the zone boundary QBZ and an energy approaching zero at Q~0.8QBZ. This mode softens with decreasing temperature, identifying the structural transformation as a continuous displacive transition.
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Ernst et al. (1987) studied this question.
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