Phonons in Liquid Bismuth Studied by Neutron Inelastic Scattering

Inelastic scattering of cold neutrons in liquid and solid polycrystalline bismuth was studied using the cold neutron facility at FiR 1 reactor in Helsinki. 5-MeV neutrons were incident on the sample and scattering angles ranging from 7.5° to 30° were used. The liquid sample used was at 280°G and the solid sample at 262°C. Similar peaks in the scattered intensity were observed both for liquid and solid samples. These were attributed to the coherent one-phonon scattering, thus leading to construction of a (ω, Q) relation similar to a dispersion curve, averaged over all possible directions of wavevectors. Both transverse and longitudinal branches of the acoustic modes were observed in the liquid as well as in the polycrystalline solid. Dispersion curves obtained for the liquid are similar to those of the solid, but the frequencies of the longitudinal branch are about 65% of those of the solid. This value is equal to the ratio of the sound velocities in liquid and solid bismuth. The intensity distribution of the scattered neutrons is analysed and it is shown that the peaks can be attributed to single phonon-like excitations, while contributions from higher Brillouin zones do not produce disturbing peaks. From this calculation it was further possible to conclude that the angle between the polarization vectors of the so-called transverse modes and the direction of propagation varies in the region of 70° to 110°. (author)