Evolution of spectral and structural characteristics of orthoborates Lu$_{0.99-x}$Gd$_{x}$Eu$_{0.01}$BO$_{3}$

The structure, IR absorption and luminescence spectra of Lu$_{0.99-x}$Gd$_{x}$Eu$_{0.01}$BO$_{3}$ solid solutions at (0 $\le x\le$ 0.15) were studied in the present work. Correspondence between the structure and spectral characteristics of these compounds was established. It is shown that Lu$_{0.99-x}$Gd$_{x}$Eu$_{0.01}$BO$_{3}$ orthoborates that consist of LuBO$_{3}$ having two stable structural modifications (calcite and vaterite) and GdBO$_{3}$ having only one structural modification (vaterite) form a solid solution with the calcite structure and microcrystal size of 15–20 $\mu$m at $x\le$ 0.05. With an increase in $x$, the amount of the vaterite phase increases sequentially. At $x\ge 0.1$, the entire bulk of a sample has the vaterite structure. At Gd$^{3+}$ concentrations of 0.05 $<x\le$ 0.1, Lu$_{0.99-x}$Gd$_{x}$Eu$_{0.01}$BO$_{3}$ samples are two-phase. It is demonstrated for the first time that at $x>$ 0.05 the vaterite phase arises both in the bulk of large microcrystals (15–20 $\mu$m) and as small microcrystals (1–2$\mu$m).