Phase transformations and photoluminescence features of Gd$_{2(1-x)}$Tb$_{x}$Eu$_{x}$O$_{3}$ solid solutions

Solid solutions Gd$_{2(1-x)}$Tb$_{x}$Eu$_{x}$O$_{3}$ (where $x$ = 1 and 2.5 mol%) are prepared by the sol–gel method with subsequent annealing at 700–1200$^{\circ}$C in air and hydrogen atmospheres. It is found by XRD and IR spectroscopy that the solutions undergo the chemical and phase transformations from the cubic modification to the monoclinic one in the annealing process and change the degree of crystallinity. Microinclusions of the Tb$_{7}$O$_{12}$ : Eu$^{3+}$ phase are present in a solid solution under certain annealing conditions, which are not formed in a hydrogen atmosphere or with a high activator concentration of 2.5 mol% at 1200$^{\circ}$C. In accordance with these transformations, the Tb$^{3+}$ and Eu$^{3+}$ photoluminescence activators are redistributed over the $C_{3i}$ centrosymmetric and $C_2$ noncentrosymmetric crystal lattice sites and the $C_s$ surface states. These rearrangements give rise to changes in the parameters of the photoluminescence spectra, which are associated with a change in the ways of transferring the excitation energy of the main radiative transitions of the Tb$^{3+}$ and Eu$^{3+}$ photoluminescence activators.