Correlated static atomic displacements and transmission-electron-microscopy contrast in compositionally homogeneous disordered alloys

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Abstract
We investigate several consequences of the atomic size effect in crystalline III-V alloys by performing numerical calculations in the framework of the extended valence-force-field model and we compare our results with available experimental data. First, we demonstrate the existence in these alloys, even if they are fully disordered, of large anisotropic spatial correlations between the static atomic displacements from the sites of the average sublattices, and we calculate the corresponding correlation functions. Second, we show that these very correlations induce a previously unrecognized characteristic contrast in the conventional transmission-electron-microscopy images of such alloys. We explore the behavior of this atomic-size-effect contrast upon changing the diffraction conditions. These results are obtained by simulating large alloy supercells and by subsequently performing dynamical image calculations. Finally we compare qualitatively and quantitatively our calculated contrast with the fine-scale contrasts observed in the experimental micrographs of these alloys.