Dynamical effects of the interaction between 4f electrons and optical phonons in rare-earth hydroxides, especially in Tb(OH)₃ and Nd(OH)₃

The Raman spectra of single crystals of Nd(OH)₃, Gd(OH) ₃, Tb(OH)₃ and of Y(OH)₃ have been investigated at helium temperatures and partially in external magnetic fields B<or=12 T. The Raman active optical frequencies have been assigned and the frequencies of the k=0 modes determined and correlated with the modes in the isomorphous rare-earth trichlorides. The interaction between the optical phonons and the 4f electrons becomes apparent in the magnetic phonon splitting in the zero-field frequency shift and, in favourable circumstances, in a phonon-electron anticrossing which depends on the magnetic field. In Tb(OH)₃ all these effects can be observed simultaneously. In the same crystal the magnetic phonon splitting occurs spontaneously below its ferromagnetic Curie temperature. All effects can be treated theoretically using a Jahn-Teller type interaction Hamiltonian bilinear in the electron and phonon variables. The numerical values of the coupling constants have been found to agree for the different interaction effects. There are indications of a systematic variation of these numbers with the type of rare-earth ion and the phonon mode considered. The contribution of optical phonons to the effect of interionic energy transfer in Tb(OH) ₃ by virtual phonon exchange is considered and compared with the aforementioned results.