Spectral and structural characteristics of Lu$_{1-x-y}$Ce$_{x}$Tb$_{y}$BO$_{3}$ orthoborates prepared by the hydrothermal synthesis method

The structure, IR absorption spectra, morphology, and photoluminescence spectral characteristics of Lu$_{1-x-y}$Ce$_{x}$Tb$_{y}$BO$_{3}$ solid solutions prepared by the hydrothermal synthesis method have been investigated. According to the X-ray powder diffraction data, the Lu$_{1-x-y}$Ce$_{x}$Tb$_{y}$BO$_{3}$ samples hydrothermally synthesized at a temperature $T$ = 200$^\circ$C have the vaterite structure, and all the observed diffraction peaks correspond to the hexagonal phase with the space group $P$6$_{3}$/$mmc$, which is isostructural to pure lutetium borate LuBO$_3$. Annealing of these samples at $T$ = 970$^\circ$C leads to a change in the structural modification and to the transition of the samples to the monoclinic phase with the space group $C$2/$c$. It has been found that, after annealing of these samples at temperatures $T$ = 800–970$^\circ$C, the luminescence intensity of Tb$^{3+}$ ions upon excitation in the absorption band of Ce$^{3+}$ ions increases by more than two orders of magnitude and becomes much higher than that in compounds of the same composition, but prepared by high-temperature synthesis. At the same time, annealing of lutetium borate doped only with terbium does not lead to a significant change in the luminescence intensity of Tb$^{3+}$ ions. The possible reasons for a multiple increase in the luminescence intensity of terbium ions due to the annealing of the hydrothermally synthesized lutetium orthoborate samples doped with cerium and terbium ions have been discussed.