Phonon Dispersion Relations of Body-Centered Cubic Thallium and the bcc-to-hcp Martensitic Phase Transformation

Neutron-scattering measurement of the phonon dispersion relations has been carried out for the body-centered cubic phase of thallium existing above 230°C. The T 1 mode propagating in the [110] direction with the [110] polarization has been found to have extremely low energy. The low-lying mode at the zone boundary is related to the atomic displacements taking place in the bcc-to-hcp martensitic phase transformation in this crystal. The lattice instability relevant to the transformation is discussed on the basis of the interplaner force-constant analysis of the T 1 mode dispersion relation.