Low-temperature absorption spectra and electron structure of HoFe₃(BO₃)₄ single crystal

Polarized absorption spectra of HoFe₃(BO₃)₄ single crystal in the range of 8500-24 500 cm^-1 were studied as a function of temperature beginning from 2K. The ground and excited electron states of Ho³⁺ were identified. The abrupt changes of the spectra at the reorientation phase transition at 4.7K were observed. The exchange splitting of some excited states were revealed and measured. They changed at the reorientation phase transition. Several vibronic transitions were observed. The splitting of absorption lines corresponding to the C₂ local symmetry of the Ho ion was not observed. Moreover, spectra of some absorption bands correspond to splitting in the cubic crystal field. There are some absorption lines, whose polarization cannot be explained both in D₃ and C₂ local symmetries. Some lines appear or disappear as a result of the transition from the easy axis to the easy plane state of the crystal. All these observations testify to the substantial changes of the local magnetic and structural properties in the excited states of the Ho³⁺ ion and to the strong influence of the magnetic moments orientation on the polarization of the electron transitions.