Luminescence properties of solid solutions of borates doped with rare-earth ions

The structural and luminescence properties of Lu$_x$Y$_{1-x}$BO$_3$ solid solutions doped with Ce$^{3+}$ or Eu$^{+3}$ have been investigated. It has been found that the solid solutions crystallize in the vaterite phase with a lutetium concentration $x <$ 0.5. For a higher lutetium concentration $x$, the solid solutions contain an additional calcite phase with a content less than 5 wt%. The luminescence spectra are characterized by intensive impurity emission under excitation with the synchrotron radiation in the X-ray and ultraviolet spectral ranges. It has been shown that, as the lutetium concentration x in the Lu$_x$Y$_{1-x}$BO$_3$ : Ce$^{3+}$ solid solutions increases, the emission intensity smoothly decreases, which is associated with a gradual shift of the Ce$^{3+}$ 5$d$(1) level toward the bottom of the conduction band, as well as with a decrease in the band gap. It has been established that, in the Lu$_x$Y$_{1-x}$BO$_3$ : Eu$^{3+}$ solid solutions with intermediate concentrations $x$, the efficiency of energy transfer to luminescence centers increases. This effect is explained by the limited spatial separation of electrons and holes in the solid solutions. It has been demonstrated that the calcite phase adversely affects the luminescence properties of the solid solutions.