Dissertation Thomas May (10.7.96)

Anharmonische Effekte in der Gitterdynamik von Metallen

Betreuer: Prof. Dr . Dieter Strauch

Abstract

Anharmonic phonon-phonon coupling strengths have been derived from a simple phenomenological model potential in order to explain the characteristic features of BCC metals in their high-temperature phase. The strong quasielastic scattering near q=2/3[111] and 1/2 [110] in BCC-Zr and BCC-Ti is caused by anharmonic broadening, with the lifetime of these low-frequency phonons of the order of a vibrational period. These modes which are connected to the structural phase transition to the hexagonal or the HCP phase show the strongest anharmonic effects. In addition, the calculated anharmonic frequency shift is able to explain the difference between recent harmonic ab-initio calculations and experiment. The interference between one- and two-phonon neutron scattering processes leads to an unusually strong intensity variation over adjacent Brillouin zones. The successful description of both harmonic and anharmonic lattice dynamics by a simple central potential indicates that angular forces may be weak.