  We present a comparative far-infrared reflection spectroscopy study of
phonons, phase transitions, spin-phonon and electron-phonon interactions in
isostructural multiferroic iron borates of gadolinium and terbium. The behavior
of phonon modes registered in a wide temperature range is consistent with a
weak first-order structural phase transition (Ts = 143 for GdFe3(BO3)4 and 200
K for TbFe3(BO3)4) from high-symmetry high-temperature R32 structure into
low-symmetry low-temperature P3121 one. The temperature dependences of
frequencies, oscillator strengths, and damping constants of some low-frequency
modes reveal an appreciable lattice anharmonicity. Peculiarities in the phonon
mode behavior in both compounds at the temperature of an antiferromagnetic
ordering (TN = 32 K for GdFe3(BO3)4 and 40 K for TbFe3(BO3)4) evidence the
spin-phonon interaction. In the energy range of phonons, GdFe3(BO3)4 has no
electronic levels but TbFe3(BO3)4 possesses several ones. We observe an onset
of new bands in the excitation spectrum of TbFe3(BO3)4, due to a resonance
interaction between a lattice phonon and 4f electronic crystal-field
excitations of Tb3+. This interaction causes delocalization of the CF
excitations, their Davydov splitting, and formation of coupled electron-phonon
modes.
