Abstract
The structural and lattice dynamical aspects of the incommensurate phase transition in ThBr4(Tc=95K) are studied by means of elastic and inelastic neutron scattering. Above Tc the temperature-dependent softening of a low-lying optic mode of wavevector qs approximately=0.3 c* is observed. The combined analysis of the room-temperature optical spectra and of the satellite extinction rules (T<Tc) allows one to determine the irreducible representation of the soft optic branch. The corresponding eigenvector consists of a simple linear combination of B1u-type and B2g-type bromine displacements. The resulting site symmetry for the thorium ions is discussed. Below Tc, constant-Q scans, near the strong (2, 3, 0.69) satellite reflection reveal the existence of two soft excitations: a low-frequency branch with a linear-like dispersion which the authors identify as the (propagating) phase mode, and an upper branch whose temperature dependence is consistent with that expected for the amplitude mode. The predictions of elementary theory concerning the dispersion, damping and oscillator strength for each of the two soft excitations are compared with the experimental data. Effects related to possible interactions with the long-wavelength acoustic models are considered.