Studies on structural and spectroscopic properties of germanate-phosphate glasses doped with Eu³⁺ ions

The effect of introducing germanium(IV) oxide as additional glass-forming oxide in the phosphate matrix with molar composition xGeO₂-(60-x)P₂O₅-30BaO-9.5Ga₂O₃-0.5Eu₂O₃, (where x = 5, 10, 15, 20, 30, 40, 45, 50, 55) was investigated. Changes in the glass local structure examined with infrared (IR-ATR) and Raman spectroscopy are closely related to glass composition. It was observed that with increasing GeO₂:P₂O₅ molar ratio, depolymerization of the phosphate network occurs by transforming Q² units into Q¹ or Q⁰ units. The luminescence properties of germanate-phosphate systems were determined by analyzing the excitation and emission spectra. The evaluated phonon energy decreases along with increasing GeO₂:P₂O₅ molar ratio, from 1145 cm^-1 to 924 cm^-1, proving that synthesized glasses can be considered relatively low phonon energy systems. The study of relations of bands assigned to transitions ⁵D₀ → ⁷F₁ (orange) and ⁵D₀ → ⁷F₂ (red) corresponds to the correlation between local structure and spectroscopic properties of systems doped with Eu³⁺ ions. It was determined that the value of fluorescence intensity ratio R/O is higher (5.58) in samples containing higher concentration of GeO₂ (55 mol%) in the matrix. The local symmetry around Eu³⁺ ions was increasing along with rising amount of phosphate structural units in the glass systems. Acquired results indicate that incorporating germanium(IV) oxide into phosphate glass matrix allows to obtain lower phonon energy of the host. Although the relation between the phonon energy and local symmetry of Eu³⁺ ions is not evident, it could be stated that for systems with low phonon energy, the degree of symmetry of the local structure may be high.