Optical phonons and low-lying crystal-field excitations in TbF₃ and HoF₃: Electronic Davydov splitting and effects of magnetic order

Abstract

The infrared and Raman active phonons (k=0) of orthorhombic TbF₃ and of isomorphous HoF₃ have been observed and attributed to the different irreducible representations of the factor groupD _2h. Moreover we investigated crystal-field excitations of the 4f-configuration of the Rare-Earth-ions and effects of their interaction with optical phonons, especially as a function of an external magnetic field. The infrared reflection spectra of these materials not only display reststrahlen bands of electric dipolar but also of magnetic dipolar type with a characteristic shape of the latter bands.

On entering the ferromagnetic ordered state pertinent symmetry reductions (D _{2h→C 2h}) become evident in the system of electronic transitions and optical phonons in TbF₃ by a change of selection rules and a magnetization-induced transfer of transition intensities to new components of the transition dipole or of the Raman polarizability tensor. These observations are in accordance with general symmetry considerations.

The delocalization of the crystal-field excitations results in a Davydov splitting of the single-ion transitions.