Structural and spectral characteristics of Pr$_{1-x}$Lu$_x$BO$_3$ orthoborates

The structure, IR absorption and luminescence spectra of Pr$_{0.99-x}$Lu$_x$Eu$_{0.01}$BO$_3$ orthoborates synthesized at 970$^\circ$C were studied at 0 $\le x\le$ 0.99. An increase in the concentration of lutetium leads to a sequential change of the structural state of the orthoborates. At first, the orthoborates are single-phase and have an aragonite structure (0 $\le x\le$ 0.1). Then, they become two-phase and contain the aragonite and vaterite phases (0.1 $<x<$ 0.6). With a further increase in x (0.6 $<x\le$ 0.8), the compounds are single-phase with a vaterite structure, then they contain the vaterite and calcite phases (0.8 $<x\le$ 0.95), and, finally, they become single-phase with a calcite structure (0.95 $<x\le$ 0.99). An unambiguous correspondence between the structural modification and IR spectra of these compounds was established. It is shown that the emission of Eu$^{3+}$ ions is observed in samples where the concentration of europium exceeds that of praseodymium.