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

Abstract

The phonon dispersion in terbium iron borate TbFe3(BO3)4 has been measured by inelastic neutron scattering in a temperature range 180<T<350 K through the displacive structural transition at TS=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 (Λ point) at temperatures T⪆TS, in full agreement with theoretical calculations. The TA soft mode undergoes considerable broadening at the Λ point near the transition temperature that can be attributed to the anharmonic interference between transverse acoustic and optical modes.