PHASE-SEPARATION, CLUSTERING, AND FRACTAL CHARACTERISTICS IN GLASS - A MAGIC-ANGLE-SPINNING NMR SPIN-LATTICE RELAXATION STUDY

A comparative study of the Si-29 spin-lattice relaxation behavior (ind uced by trace amounts of paramagnetic dopants in the glass) in phase-s eparated Li2Si4O9 and monophasic Li2Si2O5 and Na2Si2O5 glasses has bee n made in order to understand the nature of clustering and the resulti ng intermediate-range ordering. Optically clear tetrasilicate and disi licate glasses were prepared with 500 to 2000 ppm of Gd2O3, a paramagn etic dopant. The constituent structural units (Q3 and Q4 species) in a ll tetrasilicate glasses show strong differential relaxation following a power-law behavior. This is due to preferential partitioning of Gd3 + into the lower silica (Q3-rich) regions of these glasses, indicating the presence of Q species clusters too small to produce optical opale scence (a few nm to perhaps tens of nm). Preliminary results on Li-6 s pin-lattice relaxation in these glasses support this hypothesis. Diffe rential relaxation becomes more pronounced on annealing due to growth of such clusters. No such differential relaxation was observed in the monophase disilicate glasses. For spin-lattice relaxation induced by d irect dipolar coupling to paramagnetic ions, the recovery of magnetiza tion is proportional to time as M(t) approximately t(alpha) where alph a is a function of the dimensionality D of mass distribution of the co nstituent Q species around the Gd3+ paramagnetic centers in the glass. For tetrasilicate glasses D almost-equal-to 2.62+/-0.22 and the syste m behaves as a mass fractal up to a length scale of 2 to 3 nm. D is th us equal to, within error, the theoretical value of 2.6 for an infinit e percolation cluster of one type of Q species in another. For disilic ate glasses, D almost-equal-to 3.06+/-0.18 which indicates a three-dim ensional (and thus nonfractal) mass distribution of the constituent Q species over the same length scale.