Energy transfer from Tb$^{3+}$ to Eu$^{2+}$ in Ga$_2$S$_3$ : (Eu$^{2+}$,Tb$^{3+}$) crystals

The photoluminescence of Ga$_2$S$_3$ crystals activated with Eu$^{2+}$ and Tb$^{3+}$ ions separately and with ions of both types is studied in the temperature range 77–300 K. It is established that, in the range 77–300 K, the observed broadband photoluminescence in (Ga$_2$S$_3$)$_{0.95}$ : (Eu$_2$O$_3$)$_{0.05}$ crystals with a peak at 545 nm is defined by $4f^65d-4f^7(^8S_{7/2})$ intracenter transitions in Eu$^{2+}$ ions and the photoluminescence with peaks at 492, 544, 584, 625, and 680 nm in (Ga$_2$S$_3$)$_{0.99}$(Tb$_2$O$_3$)$_{0.01}$ crystals is due to the $5d\to{}^2F_j$ ($j$ = 6–2) intracenter transitions in Tb$^{3+}$ ions. It is shown that the photoluminescence bands of Tb$^{3+}$ ions in the (Ga$_2$S$_3$)$_{0.94}$(Eu$_2$O$_3$)$_{0.05}$(Tb$_2$O$_3$)$_{0.01}$ crystals disappears because of excitation energy transfer from Tb$^{3+}$ ions to Eu$^{2+}$ ions; i.e., the Tb$^{3+}$ ion is a sensitizer of the photoluminescence of the Eu$^{2+}$ ion.