Evolution of the spectral and structural characteristics of borates formed during the interaction of lanthanum and scandium oxides with a potassium tetraborate melt

The structure, morphology, IR spectra, as well as luminescence spectra and luminescence excitation spectra of europium-doped borates formed during the interaction of lanthanum and scandium oxides with a potassium tetraborate melt at 970$^\circ$C with the general formula La$_{0.99-x}$Sc$_x$Eu$_{0.01}$BO$_3$ were studied. It is shown that with an increase in the Sc$^{3+}$ concentration, three compounds are successively formed: LaBO$_3$, LaSc$_3$(BO$_3$)$_4$, and ScBO$_3$. At 0 $\le x\le$ 0.26, the samples are single-phase and have the structure of LaBO$_3$ aragonite. Within the range of 0.26 $<x<$ 0.75, the samples are two-phase and contain the structures of LaBO$_3$ aragonite and LaSc$_3$(BO$_3$)$_4$ huntite. At 0.75 $\le x\le$ 0.85, LaSc$_3$(BO$_3$)$_4$ is observed. Within the range of 0.85 $<x\le$ 0.97, the samples are two-phase and consist of LaSc$_3$(BO$_3$)$_4$ huntite and ScBO$_3$ calcite. At 0.97 $<x\le$ 0.99, the ScBO$_3$ samples have a calcite structure. Correspondence between the structure and spectral characteristics of these compounds was established.