Isomorphous phase transition of 1,2,4,5-tetrabromobenzene jumping crystals studied by Brillouin light scattering

• Isomorphous phase transition of 1,2,4,5-tetrabromobenzene (TBB).
• Brillouin light scattering of jumping crystals.
• The β–γ phase transition.
• Softening of transverse acoustic mode.
• Elastic instability and intermolecular anharmonic interaction.

The isomorphous phase transition of 1,2,4,5-tetrabromobenzene (TBB) jumping crystals was studied by Brillouin light scattering. The temperature dependence of the sound velocity and the corresponding elastic constants of three acoustic modes propagating along the [110] direction were measured as a function of temperature covering the β–γ phase transition temperature for the first time. All three elastic constants showed discontinuities at the jumping transition temperature with a large thermal hysteresis. While the longitudinal and one of the two transverse acoustic modes did not exhibit any appreciable changes in both the β and the γ phase, the lowest transverse acoustic mode showed substantial softening on approaching the jumping transition from both phases. This clearly showed that the jumping transition of this molecular crystal is driven by the elastic instability and that large intermolecular anharmonic interaction is associated with the molecular motions in the (110) plane.