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

A new effect originating from the crystal-field-excitation – phonon coupling was observed (in the far infrared spectra of a multiferroic PrFe 3 (BO 3 ) 4 ). The reststrahlen band corresponding to the A 2 symmetry nondegenerate phonon mode near 50 cm -1 (1.5 THz) splits into two bands at about 100 K, well above T N = 32 K. These bands shift and narrow progressively with further lowering the temperature, demonstrating pronounced peculiarities at T N . The observed effects were explained by an interaction of the A 2 phonon mode with the 4f crystal-field electronic excitation of Pr 3+ 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.