The surface phonon dispersion on W(001) and Mo(001) measured by high-resolution inelastic He atom scattering displayed strong anomalies. In many respects, both systems behaved in quite similar ways. In both systems, temperature-induced softening occurred at wave-vectors which were smaller than the surface Brillouin zone boundary. The condensation of this mode into a static distortion of the surface for temperatures below the critical temperature for reconstruction resulted in an incommensurate periodicity of the superlattice which was reproduced in the He atom diffraction pattern. Further lowering of the temperature led to an additional phase transition by which a commensurate structure was achieved, a c(2 x 2) pattern in the case of W and a higher order commensurate one for Mo. Surface phonons on Nb(001) exhibited a so-called normal behaviour; similar to that of the hydrogen-saturated W(001) and Mo(001) surfaces.

Surface Vibrations and Reconstruction - are Phonon Anomalies Responsible for the (001)-Surface Instabilities of Some BCC Metals? E.Hulpke: Journal of Electron Spectroscopy and Related Phenomena, 1990, 54-55, 299-308