Distribution of Tb$^{3+}$ and Eu$^{3+}$ cations in the $C$-Gd$_2$O$_3$ lattice according to photoluminescence and far IR spectra

In this work, we investigated the features of the photoluminescence of $C$-Gd$_2$O$_3$ luminophores at $x$ = 1.0 and 2.5 mol%, which are associated with the distribution of Tb$^{3+}$ and Eu$^{3+}$ activator ions over the centrosymmetric $C_{3i}$ and noncentrosymmetric C$_2$ positions of cations in the bixbyite lattice, as well as in the positions at the boundaries crystallites $C_{\mathrm{S}}$. We studied the phase transformations of the samples, changes in the morphology of crystallites, photoluminescence spectra, and spectra of the far infrared region 50–600 cm$^{-1}$ with changes in the annealing modes of the initial products of the sol-gel synthesis Gd$_{2(1-x)}$Tb$_{x}$Eu$_x$(OH)$_y$(CO$_3$)$_z\cdot n$(H$_2$O) at temperatures of 900 and 1200$^\circ$C in air and hydrogen. Correlations have been established between changes in the characteristics of the samples and the parameters of their annealing. Based on the analysis of these correlations, the redistribution of activators over the indicated cation positions was determined, and a model was proposed for identifying infrared absorption bands in accordance with the localization of activators along the cationic sublattices $C_{3i}$ and C$_2$.