Observation of soft phonon mode in TbFe3(BO3)(4) by inelastic neutron scattering

The phonon dispersion in terbium iron borate ${\mathrm{TbFe}}_3{\left({\mathrm{BO}}_3\right)}_4$ has been measured by inelastic neutron scattering in a temperature range $180<T<350$ K through the displacive structural transition at $T_S=192.5$ K and studied by ab initio calculations. Significant, but not complete, softening of the transverse acoustic (TA) branch has been observed at the corner of the Brillouin zone ($\Lambda$ point) at temperatures $T⪆T_S$, in full agreement with theoretical calculations. The TA soft mode undergoes considerable broadening at the $\Lambda$ point near the transition temperature that can be attributed to the anharmonic interference between transverse acoustic and optical modes.