STRUCTURAL ROLE OF ND3+ AND AL3+ CATIONS IN SIO2 GLASS - A SI-29 MAS-NMR SPIN-LATTICE RELAXATION, AL-27 NMR AND EPR STUDY

High field strength cations such as rare earth ions are known to show strong clustering when incorporated in minor quantities (hundreds to up to a few thousands of ppm) in highly polymerized silicate liquids and glasses. In this study, formation, distribution and growth of such clusters in Nd2O3-doped SiO2 glasses have been detected by monitoring the effect of changing Nd2O3 concentration on Si-29 magic-angle spinning nuclear magnetic resonance (MAS-NMR) spin-lattice relaxation rates. The results indicate the presence of clusters of Nd ions even at the lowest doping level of 300 ppm of Nd2O3 by weight. Once the Nd2O3 concentration reaches 1400 ppm, no new clusters are found to form on further addition of Nd2O3 and only the growth of existing clusters is observed. These observations have significance for both dipolar resonant energy transfer among Nd ions and for fluorescence quenching. Electron paramagnetic resonance spectroscopy and Si-29 MAS-NMR spin-lattice relaxation studies indicate that, on doping SiO2 glass with Al2O3, paramagnetic defect sites are created. Si-29 relaxation results also show that, on co-doping the Nd2O3-doped glasses with Al2O3, a completely homogeneous distribution of the Nd ions is obtained at an Al:Nd atomic ratio of 10:1.