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

Splitting of a nondegenerate phonon mode in PrFe3(BO3)4: 4f crystal-field level between TO and LO phonon frequencies