Structural and spectral characteristics of La$_{0.99-x}$Y$_x$Eu$_{0.01}$BO$_3$ orthoborates

The structure, IR absorption, luminescence, and luminescence excitation spectra of La$_{0.99-x}$Y$_x$Eu$_{0.01}$BO$_3$ orthoborates synthesized at 970$^\circ$C were studied at 0 $\le x\le$ 0.99. An increase in x leads to a sequential change of the structural state of these compounds. At 0 $\le x\le$ 0.1, the samples are single-phase and have the aragonite structure. Within the range of 0.1 $<x\le$ 0.8, the samples are two-phase: the vaterite phase is observed along with the aragonite structure. At 0.8 $<x\le$ 0.99, the samples are single-phase and have the vaterite structure. Correspondence between the structure and spectral characteristics of these compounds was established. It is demonstrated that with an increase in the Y$^{3+}$ concentration, the vaterite phase is formed first in the bulk of microcrystals having the aragonite structure and then in the entire sample. It is shown for the first time that a band with the maximum of 469 nm is observed in the luminescence excitation spectrum (LES) of samples having the vaterite structure and is absent in samples having the aragonite structure. It is revealed that a band in the luminescence spectrum, corresponding to the $^5$D$_0$ $\to$ $^7$F$_0$ electron transition, as well as 469 nm-band in the LES, can be an indicator of the structural state of the sample.