Spectral and structural characteristics of (Lu$_{1-x}$Eu$_{x}$)$_{2}$(WO$_{4}$)$_{3}$ tungstates

Investigated are the structure, luminescence, and IR absorption spectra of (Lu$_{1-x}$Eu$_{x}$)$_{2}$(WO$_{4}$)$_{3}$ solid solutions in a wide range of Eu concentrations (0 $\le x\le$ 1). A sequential replacement of two types of crystalline phases occurs with an increase in Eu concentration. The orthorhombic phase (Pbcn space group) of tungstate solid solutions is observed at 0 $\le x<$ 0.5. A monoclinic phase ($C2/c$ space group) arises along with the orthorhombic one in the range of 0.5 $\le x\le$ 0.8, while at $x>$ 0.8, the solid solution has a monoclinic structure. The correspondence between the structure and spectral characteristics of these compounds is established. A change in the structural state results in a change in luminescence spectra, as well as in the spectra of luminescence excitation of tungstates. The isotherm of water adsorption by the tungstate samples and its effect on their spectral and structural characteristics are studied in view of the wetting ability of (Lu$_{1-x}$Eu$_{x}$)$_{2}$(WO$_{4}$)$_{3}$ solid solutions. It is established that the maximum luminescence under the resonant excitation of Eu$^{3+}$ ions is observed in the samples with a monoclinic $C2/c$ structure at $x\sim$ 0.9.