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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.
Bibliographical noteFunding Information:
The work at the University of Minnesota was supported primarily by the Arnold and Mabel Beckman Foundation in the form of a Beckman Young Investigator Award and in part by a 3M Nontenured Faculty Award under Award Number 13673369. In addition, acknowledgment is made to the Donors of the American Chemical Society Petroleum Research Fund for partial support of this work under Award Number 53116- DNI7. Part of this work was carried out in the College of Science and Engineering Characterization Facility, University of Minnesota, which has received capital equipment funding from the NSF through the UMN MRSEC program under Award Numbers DMR-0819885 and DMR-1420013. Part of this work was carried out in the College of Science and Engineering Minnesota Nano Center, University of Minnesota, which receives partial support from NSF through the NNIN program. Work at ORNL was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Science and Engineering Division. We thank Rafael Fernandes for comments and suggestions and for reading the manuscript. We thank Jason Myers, Kevin Roberts, and Wei Zhang for assistance within the University of Minnesota shared facilities.
© 2016 American Chemical Society.