Spectral signatures of spin-phonon and electron-phonon interactions in multiferroic iron borates

High-resolution far-infrared reflection and polarized ellipsometry, as well as Raman scattering temperature-dependent measurements are used to study spin-phonon and electron-phonon interactions in rare-earth (RE) iron borates with the R32 structure of a natural mineral huntite, namely, in RFe3(BO3)4 with R=Pr, Nd, and Sm. Pronounced peculiarities in the ω(T) dependences at the Néel temperature TN≈32 K are observed for all the compounds studied and the origin of these peculiarities is discussed. A coupling between lattice phonons and crystal-field excitations of a RE ion manifests itself by a renormalization of frequencies and intensities of coupled modes. Modeling of the spectra has revealed the value of about 15 cm−1 for the electron-phonon coupling constant in PrFe3(BO3)4.