Structural specific features of solid solutions Nd$_{x}$Gd$_{1-x}$Cr$_{3}$(BO$_{3}$)$_{4}$

Optical investigations of crystals of solid solutions Nd$_{x}$Gd$_{1-x}$Cr$_{3}$(BO$_{3}$)$_{4}$, 0.01 $\le x\le$ 1, have been performed. Absorption spectra in the range of wavenumbers 11 000–15 000 cm$^{-1}$ recorded at cryogenic temperatures (16 K) reveal a significant effect of the concentration of Nd$^{3+}$ ions in the structure on the absorption in the near infrared range of the spectrum due to $f$–$f$ transitions in the Nd$^{3+}$ ion. It has been found that, at a concentration of $х>$ 0.6, two nonequivalent Nd$^{3+}$ centers are observed in the absorption spectra, which is explained by the presence of two polytypic modifications.