Energy transfer processes in Sr$_3$Y(PO$_4$)$_3$ eulytite singly doped and co-doped with Tb$^{3+}$ and Tm$^{3+}$

In this work the optical spectroscopy and the energy transfer processes involving the Tb$^{3+}$ and Tm$^{3+}$ ions, have been studied in eulytite double phosphate hosts of the type Sr$_3$Y(PO$_4$)$_3$ doped with various amounts of the two lanthanide ions. It has been found that several energy transfer and cross-relaxation processes are active in this class of materials, upon excitation in the $^5D_4$ level of Tb$^{3+}$, and in the $^1G_4$ one of Tm$^{3+}$. In particular, a Tb$^{3+}$ $\to$ Tm$^{3+}$ transfer of excitation has been found to quench strongly the $^5D_4$ level of Tb$^{3+}$. This process occurs with a transfer efficiency increasing from 0.08 to 0.62, for a donor concentration of 2 mol%, and an acceptor concentration increasing from 2 to 15 mol%. The emission spectra are strongly affected by the presence of Tb$^{3+}$ $\to$ Tm$^{3+}$ energy transfer, and Tm$^{3+}$ $\to$ Tm$^{3+}$ cross relaxation processes.