Kikuchi diffraction

Atlas of Backscattering Kikuchi Diffraction Patterns D J Dingley, K Z Baba-Kishi and V Randle ISBN 0 7503 0212 7... 

Other names instead of EBSD are Backscatter Kikuchi Diffraction (BKD), Electron Backscatter Pattern Technique (EBSP), Orientation Imaging Microscopy (OIM ), or Automated Crystal Orientation Mapping (ACOM). In combination with electrochemical studies only ex situ applications are possible. 

CONTEXT X-ray diffraction is the most common method for determining molecular structures within a crystal, but other methods are capable of faster, less detailed information about the crystal. For example, electron backscatter diffraction (also called backscatter Kikuchi diffraction), from a scanning electron microscope, measures the diffraction patterns of electrons that scatter off more than one plane in the crystal. From the patterns, the crystallographic point group, the orientation of the crystal, and the exposed Miller indices of the surface can be determined. Copper crystals, which have the advantage of simple structure, have been used to test the strengths and limitations of this method. 

Since its development in the 1970s and 1980s, the electron backscatter diffraction (EBSD) technique has become the most widely used method for micro-texture investigations in recent years [150,151]. The EBSD system is usually attached to a Scanning Electron Microscope. By arranging the specimen at an appropriate angle, an electron diffraction pattern can be generated and captured on film, a camera or a screen. The diffraction pattern is called a Kikuchi diffraction pattern, which consists of pairs of parallel lines, each of... 

The Whole-Pattern symmetry is the symmetry which takes into account all the features present on a high S5mimetry zone axis diffraction pattern (i.e. the disks, the lines inside the disk and the Kikuchi lines). As mentioned above, in order to identify a 3D S5mimetry, the pattern should, at least, display the First-Order Laue Zone. In the example given on figure 2a, this FOLZ is weak, but clearly visible and the Whole Pattern displays a 3D-4mm S5munetry. 

Electron backscatter diffraction (EBSD) — The focused electron beam of Scanning Electron Microscopes (SEM) can be used to detect the crystallographic orientation of the top layers of a sample. The backscattered electrons (information depth 40-70 nm at 25 kV accelerating potential, lateral resolution around 200 nm) provide characteristic diffraction patterns (Kikuchi lines) on a phosphor screen. The patterns are recorded by a CCD-camera and interpreted by software. The position of the unit cell of the sample is determined by the corresponding Euler angles. In scanning mode, the software produces a surface orientation mapping that consists of... 

Figure 3 Examples of (a) selected area electron diffraction, (b) zone-axis CBED pattern, and (c) off-zone axis CBED pattern showing Kikuchi lines in the diffraction pattern...

When the electrons impinge on the crystalline sample, they interact with individual lattice planes. When these interactions satisfy the Bragg condition, they exhibit backscattering diffraction and (due to the tilted sample) are directed toward a phosphor screen where the fluorescent pattern is detected by a CCD camera. The resulting pattern consists of a large number of intersecting bands, known as Kikuchi lines, which represent the unique crystallographic properties of the crystal... 

Figure 3.14. Kikuchi lines in an electron diffraction pattern of quartz. g2 = 2420 is close to the exact Bragg angle. Compare with Figure 3.16(e, f).

The corresponding SAD pattern of spots and Kikuchi lines is shown in Figure 3.16(b). The spacing between adjacent spots of the systematic row is X, and the Kikuchi lines Dj and Ex pass through the diffraction spots O and g, respectively. The second-order Kikuchi lines and Ei pass midway between —g and O and between g and Ig, respectively, and hence are a distance lx apart. 

Figure 3.16. Ewald sphere diagrams and the corresponding diffraction patterns showing the positions of the Kikuchi lines relative to the main Bragg beam, (a, b) jg = 0 (c, d) Sg < 0 and (e, f) = 0.

It is instructive to estimate the smallest tilt angle A0 which can be measured from displacements of Kikuchi lines. If we assume that a displacement of Ax = O.lx can just be measured, then from Eq. (3.59), A0 = (0.1)20, which is equal to about 0.05 degree with d = 0.5 nm and X = 0.004 nm. Thus, the accuracy with which an orientation can be determined from a diffraction pattern is greatly increased if Kikuchi lines are present. 

The nature and origin of Kikuchi lines that arise from planes of the ZOLZ were discussed in Section 3.9. Kikuchi lines can also arise from HOLZ planes and are observed outside the diffraction disks of a CBED pattern. However, within the disks there are the so-called HOLZ lines, which are continuous with the HOLZ Kikuchi lines. 

Following such a concentration of attention on ideal systems, particularly CO on Ni, it is refreshing to find that photoemission can produce structural information in the real world also. Photoelectron diffraction effects in emission from substrate-core levels at polar angles >20° can be described semi-quantitatively in terms of Kikuchi bands, which are observed, for example, in FEED experiments at 5 300 eV. 

Kikuchi lines are pairs of parallel lines consisting of one bright and one dark line in the diffraction mode as shown in Figure 3.35. Kikuchi lines are named after the Japanese scientist, Kikuchi, who discovered them in 1928. Kikuchi lines appear when the selected area for diffraction is moved to a thicker section in the specimen where the diffraction spots become weaker, or even disappear. 

Kikuchi lines appear after multiple electron diffraction in TEM and are very useful to the operator in finding the orientation of a specimen. In order to understand Kikuchi lines it is necessary to understand diffraction and reciprocal space. 

Fig- 5 Kikuchi lines. The process is visualized as a labyrinthine game. The electron is the ball, bouncing in a variety of ways and emerging unpredictably after it has undergone diffraction. The spots form a pattern without any particular order, similar to the kikuchi lines. The student claimed not to have understood what the kikuchi lines are... 

Takagi, Y., Kikuchi, T., Mizutani, T., Imafuku, M., Sasaki, S., and Mori, T., Upgrade of triple-axis/four-circle diffractometer at PF-BL3A, Rev. Sci. Instrum. 66,1802,1995. Huang, T.C., Toney, M.F., Brennan, S., and Rek, Z., Analysis of cobalt-doped iron oxide thin films by synchrotron radiation. Thin Solid Films 154, 439, 1987. Scherrer, R, Estimation of Size and Internal Structure of Colloidal Particles by Means of Rontgen Rays, Gdttinger Nachrichten 2, 98, 1918. King, H.P and Alexander, L.E. X-ray diffraction procedures, WUey, New York, 1954. 

The neutron diffraction study of Atoji (1981b) on the crystal and magnetic structures of the cubic ErCo.6 compound in the temperature range 1.6-296 K confirmed the previously reported results (Spedding et al. 1958, Lallement 1966). Its cubic structure is illustrated in fig. 9, in trigonal coordinates in order to compare it with the trigonal structure of R2C (Atoji and Kikuchi 1969). 

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