IMAGE SIMULATION OF PARTIALLY AMORPHOUS MATERIALS

The extent to which the aperiodic character of HRTEM images corresponds to amorphous domains in partially amorphous materials has not been determined. Neither the spatial nor the quantitative correspondence between the HRTEM image and the actual structure has been quantified. This complicates the complete interpretation of HRTEM images wherever amorphous domains are present. In this study, periodic continuation has been employed to model large-scale defects (i.e. amorphous domains) within crystalline zircon (ZrSiO4) in order to determine the spatial and quantitative accuracy of the phase-contrast imaging process when applied to partially amorphous materials. HRTEM image simulations of partially amorphous zircon show that increasing proportions of laterally homogeneous layers of amorphous material contribute aperiodic image intensity non-linearly, such that the apparent accumulation of amorphous material is compressed relative to the actual accumulation. Up to thirty percent of a sample can be amorphous before observable disruption to the periodicity occurs in the HRTEM image, and complete loss of the periodic character occurs when the crystalline volume fraction drops below twenty percent. Within this range of amorphous volume fractions, images exhibit a partially periodic character; however, the exact extent of the structural transition which gives rise to partially periodic images depends on sample thickness. Increasing crystal thickness decreases the image sensitivity to amorphous volumes. Laterally discontinuous regions of amorphous material contribute an aperiodic image character which corresponds spatially to the amorphous volume only when the crystalline/amorphous (C/A) interface is vertical (i.e. parallel to the electron beam). Non-vertical C/A interfaces give rise to gradational changes from periodic to aperiodic image character which are consistent with the changing columnar amorphous volume fraction across the interface. This may give rise to significant errors in the volumetric estimate of amorphous material derived from analysis of HRTEM images of lateral inhomogeneous, partially amorphous samples. Proper analysis of the image character in areas exhibiting mixed periodic/aperiodic character, however, does allow an accurate determination of the amorphous volume fraction for materials which are 30 to 80 volume percent amorphous.