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The molecular crystal 1,2,4,5-tetrabromobenzene (TBB) has been studied using Brillouin scattering and lattice dynamical calculations. A method for mapping acoustic mode anharmonicity is developed that exhibits substantial directional behavior and correlates well with the directions of the molecular movements associated with the phase transition. It is shown that by comparing the calculated sound velocities with the experimental values, insight can be gained into the displacive phase transition and the lattice dynamics of TBB. The sound velocities are plotted for three crystallographic planes containing the crystal axes. The relationships between sound velocities and lattice dynamics are discussed. The phonon dispersion curves for three directions are also presented. The Journal of Chemical Physics is copyrighted by The American Institute of Physics.