Microscopy, 2014, Vol. 63, No. S1
POSTER SESSION Dislocation imaging for orthopyroxene using an atom-resolved scanning transmission electron microscopy
Dislocations, one-dimensional lattice defects, appear as a microscopic phenomenon while they are formed in silicate minerals by macroscopic dynamics of the earth crust such as shear stress. To understand ductile deformation mechanisms of silicates, atomic structures of the dislocations have been examined using transmission electron microscopy (TEM). Among them, it has been proposed that {100} primary slip system of orthopyroxene (Opx) is dissociated into partial dislocations, and a stacking fault with the clinopyroxene (Cpx) structure is formed between the dislocations. This model, however, has not been determined completely due to the complex structures of silicates. Scanning transmission electron microscopy (STEM) has a potential to determine the structure of dislocations with single-atomic column sensitivity, particularly by using high-angle annular dark field (HAADF) and annular bright field (ABF) imaging with a probing aberration corrector.[1] Furthermore, successive analyses from light microscopy to atom-resolved STEM have been achieved by focused ion beam (FIB) sampling techniques.[2] In this study, we examined dislocation arrays at a low-angle grain boundary of ∼1° rotation about the b-axis in natural deformed Opx using a simultaneous acquisition of HAADF/ABF (JEM-ARM200F, JEOL) equipped with 100 mm2 silicon drift detector (SDD) for energy dispersive X-ray spectroscopy (EDS). Figure 1 shows averaged STEM images viewed along the b- axis of Opx extracted from repeating units. HAADF provides the cationsite arrangement, and ABF distinguishes the difference of slightly rotated SiO4 tetrahedron around the a- axis. This is useful to distinguish the change of stacking sequence between the partial dislocations. Two types of stacking faults with Cpx and protopyroxene
Fig. 1. (a) HAADF and (b) ABF of Opx view of [010] direction with inset simulation images and models of its unit cell (a = 0.52, c = 1.83 nm).
References 1. Shibata N., Chisholm MF., Nakamura A., Pennycook SJ., Yamamoto T., Ikuhara Y., Science 316 (2007) 82. 2. Kogure T., Raimbourg H., Kumamoto A., Fujii E., Ikuhara Y., Earth, Planets and Space 66 (2014) 84. doi: 10.1093/jmicro/dfu063
Direct observations of local electronic states in an Al-based quasicrystal by STEM-EELS Takehito Seki and Eiji Abe Dept. of Materials Engineering & Science, University of Tokyo 7-3-1 Hongo, Bunkyo-ku, Tokyo, Japan Most quasicrystals (QCs) reveal pseudogaps in their density of states around Fermi level, and hence the stability of QCs have been discussed in terms of energetic gains in electron systems. In fact, many QCs have been discovered by tuning valence electron density based on Hume-Rothery rule. Therefore, understanding electronic structures in QCs may provide an important clue for their stabilization mechanism. Generally, it has been frequently discussed based on an interaction between Fermi surface and Brillouin zone boundary within the framework of nearly free electron model, which is believed to be an underlying physics of a Hume-Rothery’s empirical criteria. However the hybridization effect also stabilize electron system, particularly in Al-transition metal system, in which a lot of quasicrystalline phases were discovered. Therefore, the electronic structures of QCs have not yet been fully understood, whereas their atomic structures have been studied well in terms of configuration entropy by scanning transmission electron microscopy (STEM) [1]. In the present work, we investigate local electronic states in Al-based QCs using electron energy loss spectroscopy (EELS) combined with STEM, by which EELS spectra with sub-Å probe and atomic structure can be obtained simultaneously. We report STEM-EELS results on AlCuIr decagonal phases [2]. Principal components analysis clearly shows up the atomic-site dependence of plasmon loss spectra in a two-dimensional map. Qualitatively, there seems to be certain correlations between the plasmon peaks and the core-loss edges, Al L1, Ir O23, Ir N67 and Cu L23, all of which reveal different behaviors at the cluster centers and the edges (Fig. 1). All results indicate the cluster centers have metallic states and the cluster edges have covalent states in comparison. First-principles calculations confirm the unusual electronic state. On the basis of the calculated DOS and charge-density map, we conclude Al-Ir pairs, which are mainly located at the cluster edges, hybridize their orbitals and Cu atoms are localized at cluster center without hybridization. We analyze a distribution of the hybridized orbitals by Fourier transformation of electron localization function. The distribution seems like a 10-fold standing wave with Fermi wave length. It suggests that the Hume-Rothery mechanism works even when hybridization effect mainly contributes to pseudogap formation. In other words, global distribution of hybridized orbitals is important to stabilize structures whereas usually only local atomic configurations are discussed.
Downloaded from http://jmicro.oxfordjournals.org/ at University of Lethbridge on November 14, 2015
Akihito Kumamoto1, Toshihiro Kogure2, Hugues Raimbourg3, and Yuichi Ikuhara1 1 Institute of Engineering Innovation, School of Engineering, The University of Tokyo, 2Department of Earth and Planetary Science, Graduated School of Science, The University of Tokyo, and 3Institute des Sciences de la Terre d’Orleans, University of Orleans, France
(Ppx) structures were identified between three partial dislocations. Furthermore, Ca accumulation in M2 (Fe) site around the stacking faults was detected by STEM-EDS. Interestingly, Ca is distributed not only in these stacking faults but also Opx matrix around the faults.
POSTER SESSION Dislocation imaging for orthopyroxene using an atom-resolved scanning transmission electron microscopy
Dislocations, one-dimensional lattice defects, appear as a microscopic phenomenon while they are formed in silicate minerals by macroscopic dynamics of the earth crust such as shear stress. To understand ductile deformation mechanisms of silicates, atomic structures of the dislocations have been examined using transmission electron microscopy (TEM). Among them, it has been proposed that {100} primary slip system of orthopyroxene (Opx) is dissociated into partial dislocations, and a stacking fault with the clinopyroxene (Cpx) structure is formed between the dislocations. This model, however, has not been determined completely due to the complex structures of silicates. Scanning transmission electron microscopy (STEM) has a potential to determine the structure of dislocations with single-atomic column sensitivity, particularly by using high-angle annular dark field (HAADF) and annular bright field (ABF) imaging with a probing aberration corrector.[1] Furthermore, successive analyses from light microscopy to atom-resolved STEM have been achieved by focused ion beam (FIB) sampling techniques.[2] In this study, we examined dislocation arrays at a low-angle grain boundary of ∼1° rotation about the b-axis in natural deformed Opx using a simultaneous acquisition of HAADF/ABF (JEM-ARM200F, JEOL) equipped with 100 mm2 silicon drift detector (SDD) for energy dispersive X-ray spectroscopy (EDS). Figure 1 shows averaged STEM images viewed along the b- axis of Opx extracted from repeating units. HAADF provides the cationsite arrangement, and ABF distinguishes the difference of slightly rotated SiO4 tetrahedron around the a- axis. This is useful to distinguish the change of stacking sequence between the partial dislocations. Two types of stacking faults with Cpx and protopyroxene
Fig. 1. (a) HAADF and (b) ABF of Opx view of [010] direction with inset simulation images and models of its unit cell (a = 0.52, c = 1.83 nm).
References 1. Shibata N., Chisholm MF., Nakamura A., Pennycook SJ., Yamamoto T., Ikuhara Y., Science 316 (2007) 82. 2. Kogure T., Raimbourg H., Kumamoto A., Fujii E., Ikuhara Y., Earth, Planets and Space 66 (2014) 84. doi: 10.1093/jmicro/dfu063
Direct observations of local electronic states in an Al-based quasicrystal by STEM-EELS Takehito Seki and Eiji Abe Dept. of Materials Engineering & Science, University of Tokyo 7-3-1 Hongo, Bunkyo-ku, Tokyo, Japan Most quasicrystals (QCs) reveal pseudogaps in their density of states around Fermi level, and hence the stability of QCs have been discussed in terms of energetic gains in electron systems. In fact, many QCs have been discovered by tuning valence electron density based on Hume-Rothery rule. Therefore, understanding electronic structures in QCs may provide an important clue for their stabilization mechanism. Generally, it has been frequently discussed based on an interaction between Fermi surface and Brillouin zone boundary within the framework of nearly free electron model, which is believed to be an underlying physics of a Hume-Rothery’s empirical criteria. However the hybridization effect also stabilize electron system, particularly in Al-transition metal system, in which a lot of quasicrystalline phases were discovered. Therefore, the electronic structures of QCs have not yet been fully understood, whereas their atomic structures have been studied well in terms of configuration entropy by scanning transmission electron microscopy (STEM) [1]. In the present work, we investigate local electronic states in Al-based QCs using electron energy loss spectroscopy (EELS) combined with STEM, by which EELS spectra with sub-Å probe and atomic structure can be obtained simultaneously. We report STEM-EELS results on AlCuIr decagonal phases [2]. Principal components analysis clearly shows up the atomic-site dependence of plasmon loss spectra in a two-dimensional map. Qualitatively, there seems to be certain correlations between the plasmon peaks and the core-loss edges, Al L1, Ir O23, Ir N67 and Cu L23, all of which reveal different behaviors at the cluster centers and the edges (Fig. 1). All results indicate the cluster centers have metallic states and the cluster edges have covalent states in comparison. First-principles calculations confirm the unusual electronic state. On the basis of the calculated DOS and charge-density map, we conclude Al-Ir pairs, which are mainly located at the cluster edges, hybridize their orbitals and Cu atoms are localized at cluster center without hybridization. We analyze a distribution of the hybridized orbitals by Fourier transformation of electron localization function. The distribution seems like a 10-fold standing wave with Fermi wave length. It suggests that the Hume-Rothery mechanism works even when hybridization effect mainly contributes to pseudogap formation. In other words, global distribution of hybridized orbitals is important to stabilize structures whereas usually only local atomic configurations are discussed.
Downloaded from http://jmicro.oxfordjournals.org/ at University of Lethbridge on November 14, 2015
Akihito Kumamoto1, Toshihiro Kogure2, Hugues Raimbourg3, and Yuichi Ikuhara1 1 Institute of Engineering Innovation, School of Engineering, The University of Tokyo, 2Department of Earth and Planetary Science, Graduated School of Science, The University of Tokyo, and 3Institute des Sciences de la Terre d’Orleans, University of Orleans, France
(Ppx) structures were identified between three partial dislocations. Furthermore, Ca accumulation in M2 (Fe) site around the stacking faults was detected by STEM-EDS. Interestingly, Ca is distributed not only in these stacking faults but also Opx matrix around the faults.
