  A new effect originating from the crystal-field-excitation - phonon coupling
was observed (in the far infrared spectra of a multiferroic PrFe3(BO3)4). The
reststrahlen band corresponding to the A2 symmetry nondegenerate phonon mode
near 50 cm-1 (1.5 THz) splits into two bands at about 100 K, well above TN = 32
K. These bands shift and narrow progressively with further lowering the
temperature, demonstrating pronounced peculiarities at TN. The observed effects
were explained by an interaction of the A2 phonon mode with the 4f
crystal-field electronic excitation of Pr3+ whose frequency falls into the TO -
LO frequency region of the phonon mode. Inversion of the TO and LO frequencies
for the electronic excitation and a formation of coupled electron-phonon modes
are discussed. Fitting of the TO frequency vs temperature experimental plots by
theoretical curves revealed the value 14.6 cm-1 for the electron-phonon
coupling constant.
