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

Glasses in the system xGd(2)O(3)center dot(100-x)[GeO(2)center dot V(2)O(5)] with 0 <= x <= 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 [VO(4)] structural units which contain non-bridging oxygens necessary for the charge compensation. The introduction of the excess of oxygen yields the formation of [VO(5)] structural units. This attains maximum value at 5 mol% Gd(2)O(3), in agreement with the density measurements. Further, the addition of the surplus of oxygen implies the transformation of [VO(5)] to [VO(4)] structural units and the formation of VO(4)(-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% Gd(2)O(3) (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 [GeO(6)] and [GeO(4)] structural units. This leads to the splitting of the bridge modes and a multiplication of the number of these bands.