Energy transfer from Ce$^{3+}$ to Tb$^{3+}$ in yttrium and gadolinium orthoborates obtained by hydrothermal synthesis

We studied the structure, IR absorption spectra, the spectral characteristics of photoluminescence and morphology of cerium- and terbium-doped orthoborates of gadolinium and yttrium obtained by hydrothermal synthesis at 200$^{\circ}$C, as well as solid solutions of orthoborates on the basis of yttrium, gadolinium, and lutetium with composition RECe$_{0.01}$Tb$_{0.1}$BO$_{3}$ (RE = Lu$_{0.5}$Gd$_{0.39}$, Lu$_{0.5}$Y$_{0.39}$, and Y$_{0.5}$Gd$_{0.39}$). The X-ray diffraction spectrum of yttrium orthoborate Y$_{1-x-y}$Ce$_{x}$Tb$_{y}$BO$_{3}$ is described by a hexagonal lattice with space group $P6_3/m$, which, after annealing at 970$^{\circ}$C, transforms into a monoclinic lattice with space group $C2/c$. High-temperature annealing of the studied orthoborates leads to a multiple, more than two orders of magnitude, increase in the luminescence intensity of Tb$^{3+}$ ions when the samples are excited in the absorption band of cerium ions. This effect is the result of a significant increase in the concentration of Ce$^{3+}$ ions in the orthoborates at high temperatures. It is shown that the luminescence of terbium ions is due to energy transfer from Ce$^{3+}$ to Tb$^{3+}$, which proceeds with high efficiency ($\sim$85%) by the mechanism of dipole-dipole interaction between cerium and terbium.