Dynamical Scattering and Electron Channeling in Orthorhombic and Tetragonal LaFeAsO

Abstract

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 t...