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

High field strength cations such as rare earth ions are known to show strong clustering when incorporated in minor quantities (hundreds to u p to a few thousands of ppm) in highly polymerized silicate liquids an d glasses. In this study, formation, distribution and growth of such c lusters in Nd2O3-doped SiO2 glasses have been detected by monitoring t he effect of changing Nd2O3 concentration on Si-29 magic-angle spinnin g nuclear magnetic resonance (MAS-NMR) spin-lattice relaxation rates. The results indicate the presence of clusters of Nd ions even at the l owest doping level of 300 ppm of Nd2O3 by weight. Once the Nd2O3 conce ntration reaches 1400 ppm, no new clusters are found to form on furthe r addition of Nd2O3 and only the growth of existing clusters is observ ed. These observations have significance for both dipolar resonant ene rgy transfer among Nd ions and for fluorescence quenching. Electron pa ramagnetic resonance spectroscopy and Si-29 MAS-NMR spin-lattice relax ation studies indicate that, on doping SiO2 glass with Al2O3, paramagn etic defect sites are created. Si-29 relaxation results also show that , on co-doping the Nd2O3-doped glasses with Al2O3, a completely homoge neous distribution of the Nd ions is obtained at an Al:Nd atomic ratio of 10:1.