Influence of indium on the spectral and structural characteristics of Lu$_{0.98-x}$In$_{x}$Eu$_{0.02}$BO$_{3}$

The structure, IR absorption spectra, and luminescence spectra of orthoborates Lu$_{0.98-x}$In$_{x}$Eu$_{0.02}$BO$_{3}$ at 0$\le x\le$ 0.1 has been studied. It is shown that a Lu$_{1-x}$In$_{x}$BO$_{3}$ solid solution consisting of lutetium borate LuBO$_3$, which has two stable structural modifications (calcite and vaterite), and indium orthoborate InBO$_3$, which has only one structural modification (calcite), is crystallized almost completely into the calcite st-ructure at $x$ = 0.1. It is shown for the first time that structural transformations in orthoborates Lu$_{0.98-x}$In$_{x}$Eu$_{0.02}$BO$_{3}$ with an increase in the In$^{3+}$ ion concentration begin in the surface layer of microcrystals of these compounds. The amount of the calcite phase successively increases with an increase in $x$ in the sample bulk as well; for the Lu$_{0.88}$In$_{0.1}$Eu$_{0.02}$BO$_{3}$ composition, 98% of the sample volume is comprised of the calcite structure.