Comparison of the absorption spectra of Nd$^{3+}$ ions in the NdFe$_{3}$(BO$_{3}$)$_{4}$, Nd$_{0.5}$Gd$_{0.5}$Fe$_{3}$(BO$_{3}$)$_{4}$, and Ho$_{0.75}$Nd$_{0.25}$Fe$_{3}$(BO$_{3}$)$_{4}$ crystals

Polarized optical absorption spectra in the range of some $f$–$f$ transitions of Nd$^{3+}$ ions in Ho$_{0.75}$Nd$_{0.25}$Fe$_{3}$(BO$_{3}$)$_{4}$, Nd$_{0.5}$Gd$_{0.5}$Fe$_{3}$(BO$_{3}$)$_{4}$ and NdFe$_{3}$(BO$_{3}$)$_{4}$ crystals were compared at 90 K. Spectral features related to difference of local environment of Nd$^{3+}$ ions in the mentioned crystals were revealed. In the range of $^{4}I_{9/2}\to{}^{4}G_{5/2}+{}^{2}G_{7/2}$ transition of Nd$^{3+}$ ions in Ho$_{0.75}$Nd$_{0.25}$Fe$_{3}$(BO$_{3}$)$_{4}$ crystal the appearance of some absorption lines owing to the change of the local symmetry during structural transition $R32\to P3_1$21 at $\sim$ 200 K was revealed. Their intensity increases smoothly, when the temperature decreases from the transition point. Temperature dependences of lattice parameters of Ho$_{0.75}$Nd$_{0.25}$Fe$_{3}$(BO$_{3}$)$_{4}$ crystal were measured. It was found that at the transition temperature there is a discontinuous change of the lattice parameter a, which indicates a phase transition of the first order. Lattice parameter $c$ changes smoothly.