An effect of crystal tilt on the determination of ions displacements in perovskite oxides under BF/HAADF-STEM imaging mo
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Effects of crystal tilt on the determination of the relative positions of different ion columns in compounds such as ferroelectric PbTiO3 are of critical importance, because the displacements of Ti and O relative to Pb correlate directly to the spontaneous polarization and ferroelectric properties. Here a study about the effects of small-angle crystal tilt on the relative image spots positions of different ions in PbTiO3 was carried out under high angle angular dark-field (HAADF) and bright-field imaging for aberration corrected Scanning Transmission Electron Microscope. The results indicate that crystal tilt affects the relative positions of Pb, Ti, and O greatly, and the effects are proved to depend highly on crystal tilt angle and PTO thickness. HAADF image simulations on PbTiO3, SrTiO3, and SrRuO3 indicate that the difference in atomic number is a main contributor to the relative image spot position change of different ion columns when crystal tilts.
I. INTRODUCTION
In recent years, high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) imaging method has become a powerful tool in characterizing defect1,2 and interface structures of materials3,4 due to the fact that its contrast depends highly on atomic number Z in a form of Zn (n 5 1.6–1.9),5 which makes it easy to distinguish different atomic columns. Advances in aberration-corrected (S)TEM provide high-resolution HAADF-STEM imaging with more opportunities in identifying novel structures owing to its insensitivity to electron beam irradiation and ability to image a large area without contrast inversion.6,7 Particularly, the recently developed bright field (BF) and annular bright field (ABF) STEM imaging make it possible to image light atoms such as O and H.7–9 However, under STEM imaging condition, specimen deviation slightly from crystal zone axis is a main but unavoidable factor that will impact on the quality of high resolution images. For example, previous studies indicated that crystal deviating slightly from zone axis will result in remarkable contrast reduction,10–13 especially, the relative image spot positions of different atomic columns were identified to change under HAADF- and ABF-STEM imaging mode due to the electron beam propagation in the crystal is modified when crystal tilt.14,15 In contrast, one study proposed that small crystal tilt will not cause relative image spots position Contributing Editor: Rafal E. Dunin-Borkowski a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.365
change in a Si dumbbell structure under HAADF imaging mode.16 Therefore the atomic number depending effects of crystal tilt is still a problem which should be further explored. On the other hand, previous studies have revealed crystal tilt effects on HAADF and ABF imaging mode,14,15 while how the crystal tilt will affect BF imaging mode has not been involved yet. In addition, previous works focused mostly on high symmetric crystal, such as cubic SrTiO3 [100] and ZrO2 [100].14,15 However, for a noncentral
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