Lattice Dynamics of Niobium-Molybdenum Alloys

The frequency-versus-wave-vector dispersion relations for the normal modes of vibration of a series of alloys of the transition metals niobium and molybdenum have been measured at 296^, and previous measurements on the pure metals have been extended, using coherent, one-phonon scattering of thermal neutrons. The phonon dispersion relations are very different for the two pure metals, suggesting that the electronic structure, acting through the electron-phonon interaction, plays a significant role in the determination of the dynamics of these materials. The observed neutron groups corresponding to the phonons in the alloys are not significantly broader than in the pure metals. The dependence of the dispersion curves on alloy composition is found to be complicated, both the general level of frequencies and the shape of the curves changing significantly. Fourier analysis indicates that the interatomic forces in the metals are oscillatory and of long range. Suspected Kohn anomalies are observed on several branches of the dispersion curves. With the assumption of a rigid-band model, the positions of several of these anomalies correlate with the calculated electron band structure for tungsten. The dimensions of the Fermi surface obtained from this correlation are in agreement with other Fermi-surface information.