Optical absorption and Raman spectroscopy of HoFe3(BO3)4 ferroborate: experimental study with thermodynamic analysis

The vibrational and optical properties of HoFe 3 (BO 3 ) 4 single crystals have been investigated by Raman spectroscopy and optical absorption measurements. The Raman spectrum, recorded at room temperature, reveals well-defined phonon modes associated with lattice vibrations and internal modes of structural units. Intense bands in the range 1200 – 1450 cm −1 are attributed to internal vibrations of BO 3 groups, while features in the 400 – 700 cm −1 region originate from FeO 6 octahedra. The optical absorption spectrum measured in the range 190 – 1100 nm exhibits weak narrow bands in the visible and near-infrared regions, corresponding to intra-4f transitions of Ho 3+ ions, and a pronounced absorption edge in the ultraviolet region associated with O 2− → Fe 3+ charge-transfer transitions. The optical band gap is estimated to be 3.3 – 3.5 eV. A correlation between vibrational and optical properties is established, demonstrating the role of BO 3 groups and FeO 6 octahedra in determining the electronic structure and optical response of the material. The results provide a consistent experimental characterization of HoFe 3 (BO 3 ) 4 and indicate its potential for optical and magneto-optical applications.

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