Disorder in Crystals and Order in Amorphous Materials; Their Measurement by Tem and Diffraction Methods. M. M. J. Treacy and J. M. Gibson*. NEC Research Institute, Inc., Princeton, NJ 08540; *Phys & Matls. Sci, U. Illinois, Urbana-Champaign, IL 61801.

This talk discusses statistical methods in transmission electron microscopy (TEM) for studying 1-dimensional disorder in crystals, and 3-dimensional intermediate-range order in amorphous materials. A newbreed of fluctuation microscopy techniques is described.

The statistics of planar fault distributions in the intergrown FAU/EMT (cubic/hexagonal) family of zeolite materials, can be revealed directly by conventional bright-field imaging in the TEM. Diffraction simulations using the DIFFaX computer program confirm that cubic and hexagonal stacking sequences are clustered in this family of materials. The clustering is shown to be driven by relaxation of strain energy associated with twin interfaces.

Variable coherence microscopy can be used to study speckle statistics in TEM dark-field images of evaporated amorphous silicon and germanium films. Image speckle is a direct measure of 2- to 4-atom correlations in the specimen, and is found to be suppressed when films are annealed at 400 C. Modeling confirms that annealed films relax into the continuous random networks. However, as-deposited films exhibit too much speckle to be random networks, but not enough to be truly polycrystalline. Molecular dynamics simulations show that as-deposited films comprise paracrystallites, formed by strong elastic deformation of crystalline grains 1 - 3 nm diameter. The deformation is driven by the relaxation of grain boundary stresses. Paracrystallite models closely reproduce the 2-atom pair-correlation functions obtained by diffraction. This result explains why polycrystalline Si films with grain sizes below 3 nm have not been observed.