Atomic structure of YB 56 studied by digital high-resolution electron microscopy and electron diffraction
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Jan-Olov Bovin National Center for HREM, Inorganic Chemistry 2, Chemical Center, Lund University, P.O. Box 124, S-221 00 Lund, Sweden
Iwami Higashi Department of Chemistry, Chiba Institute of Technology, Narashino, Chiba 275-0016, Japan
Takaho Tanaka National Institute for Research in Inorganic Materials, Tsukuba, Ibaraki 305-0044, Japan
Yoshio Ishizawa Iwaki Meisei University, Iwaki, Fukushima 970-8551, Japan (Received 14 February; accepted 20 September 2000)
Atomic positions for Y atoms were determined by using high-resolution electron microscopy and electron diffraction. A slow-scan charge-coupled device camera which had high linearity and electron sensitivity was used to record high-resolution images and electron diffraction patterns digitally. Crystallographic image processing was applied for image analysis, which provided more accurate, averaged Y atom positions. In addition, atomic disordering positions in YB56 were detected from the differential images between observed and simulated images based on x-ray data, which were B24 clusters around the Y-holes. The present work indicates that the structure analysis combined with digital high-resolution electron microscopy, electron diffraction, and differential images is useful for the evaluation of atomic positions and disordering in the boron-based crystals.
I. INTRODUCTION
For soft x-ray synchrotrons in the 1–2-keV energy region, high-resolution and radiation resistant monochromators are required. Gadolinium gallium garnet and beryl are used for monochromators because of the dspacing of these materials. However, the crystal quality of these materials is low, and they suffer from synchrotron radiation damage. Recently, high-quality YB56 single crystals, with the YB66 structure,1–3 have been synthesized for use as high-resolution and synchrotronradiation-resistant monochromators.4,5 After the discovery of the application of YB 56 as a soft x-ray monochromator, the crystal structure of YB 5 6 (a ⳱ 2.34600 nm) has been determined using singlecrystal x-ray diffractometry.6 The boron framework of YB56 is basically made of 8 supericosahedra (B12)13 and 8 nonicosahedral B80 clusters (totally 1584 boron atoms).
a)
Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 16, No. 1, Jan 2001
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The site occupancy of yttrium in YB56 is 0.575, which suggests that the peanut-shaped “Y-sites” (a pair of yttrium atoms) in the boron clusters should be occupied in most cases by only one yttrium atom.2,3,6 The structure of YB56 –66 has been investigated by transmission electron microscopy, 3 scanning tunneling microscopy, 7 and atomic force microscopy.8 The scanning tunneling microscopy showed a periodicity of 1.2 nm, which indicates that there is no surface reconstruction at the YB66 surface. To our knowledge, no direct imaging of single yttrium atomic positions in the boron clusters has yet been done. In our previous work,9 the Y-holes in the YB56 were directly detected in the b
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