LATTICE-DYNAMICS AND SELF-DIFFUSION IN NIOBIUM AT ELEVATED-TEMPERATURES

The temperature dependence of the phonon dispersion of Nb from room te mperature up to 2223 K has been measured by means of inelastic neutron scattering. Over a temperature range of nearly 2000 K the dispersion changes only moderately. Effects due to electron-phonon interaction al most vanish at the highest temperature. Parameterizing the dispersion curves by Born-von Karman force constants, the phonon density of state s g(omega) at different temperatures was computed. Employing the model of phonon controlled diffusion in BCC metals, the migration barrier H (m) and the formation entropy S(f) have been calculated. The results a gree with those of quenching and electron irradiation experiments.