Spin–Phonon and Electron–Phonon Interactions in Multiferroic GdFe\(_{3}\)(BO\(_{3}\))\(_{4}\) and TbFe\(_{3}\)(BO\(_{3}\))\(_{4}\) Evidenced by IR Reflection Spectroscopy

We study temperature-dependent polarized reflection spectra of multiferroic GdFe\(_{3}\)(BO\(_{3})_{4}\) and TbFe\(_{3}\)(BO\(_{3})_{4}\) single crystals. Signatures of spin–lattice interactions in both compounds and of a formation of the electron–phonon coupled mode in TbFe\(_{3}\)(BO\(_{3})_{4}\) are discussed.