Comparison of the Absorption Spectra of Nd3+ Ions in the NdFe3(BO3)4, Nd0.5Gd0.5Fe3(BO3)4, and Ho0.75Nd0.25Fe3(BO3)4 Crystals

The polarized optical absorption spectra in the region of a series of the f–f transitions of Nd3+ ions in the Ho0.75Nd0.25Fe3(BO3)4, Nd0.5Gd0.5Fe3(BO3)4, and NdFe3(BO3)4 crystals at 90 K have been compared. The spectral features related to the difference in the local environment of Nd3+ ions in these crystals have been established. In the region of the transition 4I9/2 → 4G5/2 + 2G7/2 of Nd3+ ions in the Ho0.75Nd0.25Fe3(BO3)4 crystal, the appearance of some absorption lines at the structural transition R32 → P3121 around ~200 K due to the local symmetry variation has been found. The intensity of these lines smoothly increases with a decrease in temperature from the transition point. The temperature dependence of the lattice parameters of the Ho0.75Nd0.25Fe3(BO3)4 crystal has been measured. It has been found that, at the transition temperature, the lattice parameter a changes stepwise, which is indicative of the occurrence of a first-order phase transition. The lattice parameter c changes smoothly.