Identification of the chirality of intermetallic compounds by electron diffraction

  • PDF / 531,749 Bytes
  • 6 Pages / 612 x 792 pts (letter) Page_size
  • 53 Downloads / 236 Views

DOWNLOAD

REPORT


S5.8.1

Identification of the chirality of intermetallic compounds by electron diffraction S. Fujio, H. Sakamoto, K. Tanaka1 and H. Inui Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan 1 Department of Advanced Materials Science, Kagawa University, Takamatsu 761-0396, Japan ABSTRACT A new CBED method is proposed for the identification of the chirality of enantiomorphic crystals, in which asymmetry in the intensity of the reflections of Bijvoet pairs in an experimental symmetrical zone-axis CBED pattern is compared with that of a computer-simulated CBED pattern. The intensity difference for reflections of these Bijvoet pairs results from multiple scattering among relevant Bijvoet pairs of reflections, each pair of which has identical amplitude and different phase angles. With the method, a single CBED pattern is sufficient and chiral identification can be made for all possible enantiomorphic crystals that are allowed to exist in crystallography. The method is successfully applied to some chiral intermetallic compounds. INTRODUCTION Enantiomorphism is usually referred to and used to describe objects that are lacking of improper rotation (rotoinversions and rotoreflections). Because of the absence of a center of symmetry ( 1), a mirror plane (m= 2 ) and a 4 axis, such enantiomorphic (chiral) crystals or molecules can occur in two different forms that are related as a right hand and a left hand and these crystals are mirror related and are not superimposable with each other. These enantiomorphically-related crystals belong to either of the 11 crystal classes (point groups), as summarized in Table 1 [1,2]. Enantiomorph identification is usually made by X-ray diffraction methods utilizing anomalous scattering phenomenon [3,4]. However, a relatively large-sized single crystal of high quality and a sufficiently strong X-ray beam are needed in many cases [5,6]. If the distinction can be made by electron diffraction in the transmission electron microscope (TEM), such difficulties can be avoided owing to the capability of the TEM of using a nanometer-sized electron probe [7,8]. Although a few methods have so far been proposed to determine the chirality of enantiomorphic crystals such as quarts [9,10] and MnSi [11] by electron diffraction, none of them can easily be extended to all possible enantiomorphic crystals. We have recently proposed a new CBED (convergent-beam electron diffraction) method for identification of chirality of enantiomorphic crystals, in which asymmetry in the intensity of the reflections of Bijvoet pairs in an experimental symmetrical zone-axis CBED pattern is compared with that of a computer-simulated CBED pattern. The intensity difference for reflections of these Bijvoet pairs results from multiple scattering (dynamical nature of electron diffraction) among relevant Bijvoet pairs of reflections, each pair of which has identical amplitude and different phase angles. With the present method, chiral identification can be made for all the possible enantiomorp