Far infrared studies of PrFe$_{3}$(BO$_{3})_{4}$

We present results on polarized far infrared reflectance, transmittance and ellipsometry measurements of PrFe$_{3}$(BO$_{3})_{4}$ single crystals in a wide temperature range (5 - 300K). Rare-earth iron borates RFe$_{3}$(BO$_{3})_{4}$ undergo an antiferromagnetic phase transition at temperatures below 40 K and all of them demonstrate magnetoelectric and magnetoelastic effects. PrFe$_{3}$(BO$_{3})_{4}$ orders antiferromagnetically at T$_{N}$~$=$~32 K. Pronounced changes in the low-frequency phonon spectra of PrFe$_{3}$(BO$_{3})_{4}$ are observed at T$_{N}$ which points to a significant spin-lattice interaction. Below 90 K, a new feature at 48 cm$^{-1}$ appears in the pi-polarized reflectance spectra. We attribute this feature to a Pr$^{3+}$ crystal-field transition that becomes observable in reflectance due to interaction with a nearby phonon 60cm$^{-1}$.