Crystal field excitations across the isostructural transition in TbNiAl and TbPdAl

We report on the inelastic neutron scattering study performed on TbNiAl and TbPdAl with special focus on the isostructural transition that occurs for both compounds around 110 K. The measured inelastic neutron spectra revealed crystal-field excitations below 15 meV. The corresponding scattering intensity shifts gradually to lower energies when increasing temperature from 4 to 140 K that corresponds to gradual population of the excited crystal-field states. We did not observe any significant deviation from this tendency around the temperature of the structural transition, which may indicate that the crystal field potential seen by the Tb ion has a very similar strength as well as local point symmetry above and below the structural phase transition.