Dynamical Scattering and Electron Channeling in Orthorhombic and Tetragonal LaFeAsO

In the study of LaFeAsO and doped compounds thereof, high-resolution transmission electron microscopy (TEM) has been used to characterize the structural and morphological properties, while cryo-TEM has been used to purportedly observe the structural phase transition occurring at 160 K. Often, the appearance and disappearance of Bragg spots in diffraction patterns, as well as changes in diffraction contrast in bright-field images, have been pointed to as indicators of the phase transition. Here we show that effects not related to the transition can produce signatures reminiscent of those typically associated with the symmetry change. In particular, we demonstrate the effects of electron channeling and multiple scattering on intensity modulation of atomic columns and Bragg spots in high-angle annular dark-field scanning TEM (HAADF-STEM) images and parallel-beam electron diffraction (PBED) patterns, respectively, from both tetragonal and orthorhombic LaFeAsO. From electron-transparent lamellae, we quantify the spatially varying thickness and, via atomic-resolution HAADF-STEM imaging, demonstrate the thickness-dependent modulation of intensities within the (Fe, O) and (La, As) columns at 300 K. From PBED patterns and Fourier-filtered high-resolution bright-field images acquired both above and below the structural phase-transition temperature, we show how intensities of forbidden reflections are modulated by moving to regions of differing thickness, independent of a symmetry change (i.e., at a fixed temperature). The experimental results are supported with multislice simulations of thickness-dependent atomic-column contrast and Bragg-spot intensities.