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

The phonon dispersion in terbium iron borate ${\mathrm{TbFe}}_{3}{({\mathrm{BO}}_{3})}_{4}$ has been measured by inelastic neutron scattering in a temperature range $180lTl350$ K through the displacive structural transition at ${T}_{\mathrm{S}}=192.5$ 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 ($\mathrm{\ensuremath{\Lambda}}$ point) at temperatures $T⪆{T}_{\mathrm{S}}$, in full agreement with theoretical calculations. The TA soft mode undergoes considerable broadening at the $\mathrm{\ensuremath{\Lambda}}$ point near the transition temperature that can be attributed to the anharmonic interference between transverse acoustic and optical modes.