Atomic structure of amorphous solids from high resolution electron microscopy - a technique for the new millennium?

A high performance electron lens can distinguish scattering from different regions of a sample, i.e., atomic density rho (atm)(r) = Sigma (i) delta (r - R-i), with a resolution approaching similar to1 Angstrom - information w hich is not available from any other technique. Early high resolution elect ron microscopy (HREM) studies of amorphous solids faced scepticism due to t he association of 'amorphous' with 'random'. This paper demonstrates two wa ys in which HREM gives information about atomic structure, which is not ava ilable from diffraction. Firstly, while diffraction depends on pair correla tions, HREM depends on integral rho (atm) (r) dz, and hence provides a prob e for triplet correlations (for example). Secondly, while diffraction depen ds on \A(k)\ (2) (where A(k) = FT{rho (atm)(r)} is the scattering amplitude ), HREM depends on.4(k). This provides a probe for locally anisotropic medi um range order (MRO), such as in the quasi-Bragg plane model for MRO in a-S iO2 by Gaskell et al. These relationships are demonstrated using computer S imulations on models of ta-C. (C) 2001 Elsevier Science B.V. All rights res erved.