The peculiarities of the spectral characteristics of different Lu$_{1-x}$RE$_{x}$BO$_{3}$ structural modifications

The structure, IR absorption and luminescence spectra of the microcrystals of Lu$_{1-x}$RE$_{x}$BO$_{3}$, Lu$_{0.99-x}$Tb$_{x}$Eu$_{0.01}$BO$_{3}$, and Lu$_{0.99-x}$Y$_{x}$Eu$_{0.01}$BO$_{3}$ orthoborates (0 $\le x \le$ 0.25) synthesized at 970$^{\circ}$С were studied. An increase in х leads to a sequential change of the structural state of the orthoborates. At $x\le$ 0.07 – 0.09, the compounds form a solid solution with the calcite structure and microcrystal size of 8 – 20 $\mu$m. Then, they become two-phase: the vaterite phase arises along with the calcite structure. At $x\ge$ 0.2 – 0.25, the entire bulk of a sample has the vaterite structure. A correspondence between the structure and spectral characteristics of these compounds was established. Luminescence spectra were investigated at different wavelengths of exciting light. This allowed obtaining information on the structure of a near-surface layer and the bulk of microcrystals of the investigated samples. It is shown that the vaterite phase arises both in the bulk of large microsrystals (8 – 20 $\mu$m) and in the form of small microsrystals (1 – 2 $\mu$m).