Spectral characteristics of different structural modifications of Lu$_{1-x}$Eu$_x$BO$_3$

The spectral and structural characteristics of polycrystals of Eu$^{3+}$-doped lutetium borates (Lu$_{1-x}$Eu$_x$BO$_3$) annealed at different temperatures have been investigated over a wide range of europium concentrations. The conditions for the preparation of Lu$_{1-x}$Eu$_x$BO$_3$ in the calcite and vaterite phases have been determined. It has been found that there is a radical difference between the excitation spectra of the main emission bands of the calcite and vaterite phases of the Lu$_{1-x}$Eu$_x$BO$_3$ borates. The influence of the europium concentration on the structure of Lu$_{1-x}$Eu$_x$BO$_3$ has been analyzed. It has been established that, at europium concentrations of higher than 15 at%, only the vaterite structure is formed independently of the annealing temperature. Thus, by varying the Eu$^{3+}$ concentration and the annealing temperature of Lu$_{1-x}$Eu$_x$BO$_3$, it is possible to directionally synthesize a specific structural modification and, consequently, to control the spectral characteristics of this compound.