Experimental and theoretical investigations on the structure-properties interrelationship of the gadolinium-vanadate-germanate glasses

Glasses in the system xGd₂O₃·(100-x)GeO₂·V₂O₅] with 0 ≤ × ≤ 20 mol% have been prepared from the melt quenching method. In this paper, we investigated changes in germanium coordination number in gadolinium-vanadate-germanate glasses through molar volume analysis, measurements of densities, investigations of FTIR and UV-VIS spectroscopy, calculations of density functional theory (DFT). Analyzing the structural changes resulted from the IR spectra we found that the gadolinium ions have a pronounced affinity toward VO₄] structural units which contain non-bridging oxygens necessary for the charge compensation. The introduction of the excess of oxygen yields the formation of VO₅] structural units. This attains maximum value at 5 mol% Gd₂O₃, in agreement with the density measurements. Further, the addition of the surplus of oxygen implies the transformation of VO₅] to VO₄] structural units and the formation of VO₄⁻³ orthovanadate structural units. The UV-VIS spectra show a broad UV absorption band located in the 300–500 nm region. These bands are assumed to originate from the combination of vanadium ions possibly present in the three states of valence. The presence of Ge-Ge wrong bonds attains its maximum values in the samples with x = 5 and 15 mol% Gd₂O₃ (bands centered in the 250–300 nm range). DFT calculations show the massive vibrations of the VO_n] structural units coupled with each other via GeO₆] and GeO₄] structural units. This leads to the splitting of the bridge modes and a multiplication of the number of these bands.