The evolution of the spectral and structural characteristics of La$_{0.98-x}$Lu$_{x}$Eu$_{0.02}$BO$_{3}$ orthoborates

The structure, IR absorption, luminescence, and luminescence excitation spectra of La$_{0.98-x}$Lu$_{x}$Eu$_{0.02}$BO$_{3}$ orthoborates synthesized at 970$^{\circ}$C were studied at 0 $\le x \le$ 0.98. An increase in $x$ leads to a sequential change of the structural state of the orthoborates. At first, the compound has the aragonite structure. Then, it becomes two-phase and contains the aragonite and vaterite phases. With a further increase in $x$, the compounds have the vaterite structure, then the vaterite and calcite structure, and, finally, the calcite structure. 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 in the bulk of microcrystals of samples that have the aragonite structure.