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

The vibrational and optical properties of HoFe₃(BO₃)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 BO₃ groups, while features in the 400 – 700 cm⁻¹ region originate from FeO₆ 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 Ho³⁺ ions, and a pronounced absorption edge in the ultraviolet region associated with O²⁻ → Fe³⁺ 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 BO₃ groups and FeO₆ octahedra in determining the electronic structure and optical response of the material. The results provide a consistent experimental characterization of HoFe₃(BO₃)₄ and indicate its potential for optical and magneto-optical applications.

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