Spectral and structural features of Lu$_{1-x}$RE$_x$BO$_3$ compounds

The luminescence spectra, luminescence excitation spectra, IR absorption spectra, and crystal structure of orthoborates Lu$_{1-x}$RE$_x$BO$_3$ (RE = Eu, Gd, Tb, Y, Dy) have been investigated. It has been found that the solid solution consisting of a LuBO$_3$ orthoborate, which has two stable structural modifications (calcite and vaterite), and an REBO$_3$ orthoborate, which has one structural modification (vaterite), crystallizes only in the vaterite structure when the concentration of a rare-earth ion substituting for lutetium exceeds 15–20 at %. The investigation of the photoluminescence spectra has demonstrated that, for rare-earth ions Lu$^{3+}$, Eu$^{3+}$, Y$^{3+}$, and Gd$^{3+}$ in the vaterite modification of Lu$_{1-x}$RE$_x$BO$_3$ orthoborates, there are at least two positions that are not equivalent in the symmetry of the local environment. It has been established that the maximum intensity of the luminescence of the vaterite modification of Lu$_{1-x}$RE$_x$BO$_3$ compounds synthesized at 970$^\circ$C, which is observed at a terbium concentration of 15 at%, is several times higher than the maximum intensity of the luminescence of the calcite modification.