Investigation of the atomic structure and visibility of crystal/amorphous interfaces in Pd 80Si 20 alloy by HRTEM image simulations

Abstract

Atomic models of isolated crystalline Pd 3Si embedded in amorphous Pd 80Si 20, a planar crystalline Pd 3Si/amorphous Pd 80Si 20 interface and a faceted crystalline Pd 3Si/amorphous Pd 80Si 20 interface were constructed using a dense random packing (DRP) model for the amorphous alloy. Multislice programs were used to simulate high-resolution transmission electron microscope (HRTEM) images of these models for a JEOL 4000EX microscope. The simulated images show that amorphous alloy superimposed on crystalline Pd 3Si degrades the crystal image. The visibility limit of crystalline Pd 3Si embedded in amorphous Pd 80Si 20 occurs for an amorphous/crystal thickness ratio of about 3.4 near optimum defocus (- 50.0 nm), but this can be increased by a factor of two using optical diffraction. A similar visibility limit occurs for crystalline protrusions at a crystal/amorphous interface. The visibility limit is affected by defocus, and at -60.0 nm defocus a crystalline protrusion of Pd 3Si can become more visible due to a zero in the microscope contrast transfer function at the nearest-neighbor bond distance of the amorphous alloy. The atomic arrangement of crystalline Pd 3Si extends a few atomic diameters into the amorphous matrix for a crystal/amorphous interface constructed with the DRP model, and this structure can be observed by HRTEM.