Spectroscopic Observation of Fractal Packing of Oxygen in Variably Modified Glassy Tetrahedral Networks.

The spatial distribution of bridging and nonbridging oxygen atoms in silicate glass networks with a wide range of connectivity is studied using (17)O nuclear magnetic resonance spin-lattice relaxation spectroscopy. The results demonstrate a mass fractal spatial distribution of both oxygen species in the network at the nanometer length scale. The fractal dimension increases with increasing relative fraction of these oxygen species and reaches a maximum value of ∼2.6. Such locally fractal character of the structural network is key to the understanding of the apparently anomalous transport, relaxation, and localization properties that are the hallmarks of the glassy state.