J. Murray Gibson, Materials Science Div., Argonne
Glimpsing Medium-range Order - The Structure of Glass from Fluctuation Electron Microscopy
Physics Division Colloquium - 9 Feb 2001

Diffraction methods are widely used to illuminate the structure of glass and related materials, in which there is no "long-range" crystalline order. The radial distribution function, or atom-pair probability density function, is readily obtained from x-ray, neutron or electron diffraction. This function powerfully illuminates "short-range" atom ordering, which is typically strong because of chemical bonding constraints. Transmission electron microscopes can directly visualize the structure of glasses near the atomic level, but lack the resolving power to identify atom positions. We have developed a technique that analyzes the real-space fluctuations in electron diffraction from glasses and obtains higher-order atomic correlation functions. We show experimentally and theoretically that the four-atom correlation function is exquisitely sensitive to "medium-range" atomic order, beyond a few atomic separations. Although our technique can be applied widely in glassy materials, extensive studies of amorphous silicon have shown the importance of this information. We now believe that the conventional model of the structure of amorphous silicon, the continuous random network, which was obtained from diffraction, seriously underestimates the typical degree of topological order in the material. Our insight can explain some important physical properties of amorphous tetrahedral semiconductors.

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