Comparison of the Absorption Spectra of Nd.sup.3+ Ions in the NdFe.sub.3.sub.4 Crystals

The polarized optical absorption spectra in the region of a series of the f-f transitions of Nd.sup.3+ ions in the Ho.sub.0.75Nd.sub.0.25Fe.sub.3(BO.sub.3).sub.4, Nd.sub.0.5Gd.sub.0.5Fe.sub.3(BO.sub.3).sub.4, and NdFe.sub.3(BO.sub.3).sub.4 crystals at 90 K have been compared. The spectral features related to the difference in the local environment of Nd.sup.3+ ions in these crystals have been established. In the region of the transition .sup.4I.sub.9/2 [right arrow] .sup.4G.sub.5/2 + .sup.2G.sub.7/2 of Nd.sup.3+ ions in the Ho.sub.0.75Nd.sub.0.25Fe.sub.3(BO.sub.3).sub.4 crystal, the appearance of some absorption lines at the structural transition R32 [right arrow] P3.sub.121 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 Ho.sub.0.75Nd.sub.0.25Fe.sub.3(BO.sub.3).sub.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.