th  - 1987 - 51st ENC Conference

MF35
RELAXATION MECHANISMS AND EFFECTS OF MOTION IN NaAISi30 s
LIQUID AND GLASS: A HIGH TEMPERATURE NMR STUDY.
S.-B. Liu, A. Pines (Dept. of Chemistry, University of California,
Berkeley, CA 94720)
*J. F. Stebbins (Dept. of Geology, Stanford University, Stanford, CA 94305)
The dynamics of atomic or "molecular" motion in molten silicates are poorly known
"but are of central importance in the understanding of both the thermodynamic and tran-
sport properties of these complex materials. Despite severe technical problems, l~%,I~R
spectroscopy on the liquids themselves is potentially a very powerful and perhaps unique
tool for studying these dynamics. NMR lineshape mad relaxation time studies of 23Na
and ~Si at temperatures as high as 1200°C have begun to reveal the details of this mo-
tion, and in particular the local dynamical cause of the macroscopic glass transition.
For NaA1Si30 s (corresponding to the mineral albite), all 23Na T 1 data from 600 to
1200°C are above the minimum, indicating the relative high mobility of this "network
modifying" cation. The dominant spin-lattice relaxation mechanism is found to be qua-
drupolar interaction which arises from the sodium ion diffusion. The activation energy is
71-1-3 k J/tool, in close agreement with the results of electrical conductivity and Na tracer
diffusion data. The correlation time of the Na motion is estimated to be about 8.5x10-11s
at 1100°C. Relatively minor anomalies in T 1 and T 2 for 23Na are seen at the bulk glass
transition temperature, indicating that above T s, some Na motion is correlated with
structural motion of the silicate framework.
In contrast, all T 1 data for 2gSi in the same temperature range are below the
minimum, because of the relative immobility of this "network former". Most
significantly, dramatic changes in the ~Si T 1 slope, and in the relaxation mechanism,
occur at the bulk glass transition temperature, indicating a rather abrupt increase in the
extent of structural motion at the same point where abrupt changes in thermodynamic
properties occur. Both dipole-dipole interactions and the chemical shift anisotropy
known from crystals and glasses contribute to the 2QSi spin-lattice relaxation above the
glass transition, but are insufficient to completely account for the observed T 1 values.
Relaxation may therefore involve short lived distortions of the structure which cause
temporary excursions in chemical shift several times greater than those which can be
quenched into the glass.