High Resolution Electron Diffnulion Patterns from Mierocrystals 



101 



Fig. 4. Fine structures in the dilTraction pattern from indi- 

 vidual ZnO crystals, (b) Undeflected beam in the difl'raction 

 pattern of the crystal (a), (c) Central beam with fine struc- 

 tures, and (d) reflection with subsidiary maxima from other 

 patterns. (.H.T. = 80 kV.) 



Straight line drawn through the doublet. These spots 

 are deviated to larger angles than the doublet itself. 

 Their intensity is two to three orders of magnitude 

 lower than the intensity of the doublet. As the inter- 

 ference figure observed is frequently not symmetric 

 to the center of the diagram, the effect cannot be 

 interpreted as an extension of the reciprocal lattice 

 point (5) or as Fraunhofer diffraction. In both cases, 

 the crystal size obtained from the micrographs would 

 result in spots of much narrower spacing than that 

 found in the diffraction pattern. In our opinion, the 

 weak spots may be caused by beams belonging to 

 further wave fields disregarded in interference 

 double refraction. 



Some fine structures in (hkO) reflections from 

 MgO crystals are most probably subsidiary maxima, 

 caused by the extensions of the scattering amplitude 

 around each reciprocal lattice point, which arc nor- 

 mal to the boundary faces of the crystal. The fine 

 structure shown in fig. 5, a, corresponds to a crystal 

 500 A in cube length. Unfortunately the spherical 

 aberration of the objective lens introduces an uncer- 

 tainty of about 0.1 /< into the exact location of the 

 scattering area when the beams are deflected about 

 larger Bragg angles. Therefore, diffraction patterns 

 of individual crystals of this size cannot be obtained. 

 Nevertheless, the defocused diffraction pattern per- 

 mits an estimation of the upper limit of the crystal 

 size, which is O.I //. The above supposition is thus 

 confirmed indirectly. 



On defocusing a diffraction pattern, darkfield 

 shadow-images of the scattering crystal regions arc 

 obtained from the strong refiections. In the case 

 of a wedge-shaped part of a crystal, two waves and 

 correspondingly two rays are present for each rellec- 

 tion. Therefore, in the overfocused pattern, two 



Fig. 5. (a) Subsidiary maxima and (b), (c), (d) ne\s t>pcs 

 of fine structures in diffraction patterns from MgO crystals. 

 In (c) and (d) each spot of a quartet is split up into two 

 spots. 



close lying shadow images of the wedge are ob- 

 tained for each strong reflection (fig. 3). This may be 

 regarded as a "double" projection of the second- 

 stage image of the wedge, produced from the spots 

 of the corresponding double-refraction doublet, 

 which appears in the second-stage image of the dif- 

 fraction pattern. 



In the diffraction patterns of individualZnO crys- 

 tals, fine structures in the undifTracted beam have 

 also been observed (fig. 4, a, b, c). Contrary to the 

 subsidiary maxima, which are found at Bragg rctlcc- 

 tions (fig. 4, d) and have already been obtained (8). 

 they are not symmetrical to the center of the pattern. 

 For an interpretation of the effect, it is most prob- 

 able that the dynamical theory of electron diffraction 

 will have to be considered. 



Finally, some other diffraction effects were ob- 

 served, which have not been interpreted up to now 

 (fig. 5, b, c. d). The splitting up of each spot of a 

 double refraction quartet into two spots occurs 

 rather frequently, and has also been obtained by 

 Molicre (6). 



References 



1. VON Ardenne, M., Kolloid-Z. 108, 195 (1944). 



2. VON Ardenne, M., Schiebold, E., and Gunther, F., 



Z. Pliysil< 119, 352 (1942). 

 .V BorRsni, H., .)/;/;. Pliysik (5) 27, 75 (1936). 



4. Hill II R. J. and IUktr. R. I-.. J. Appl. Plivs. 17, 12 



(1946). 



5. VON Laue, M.. Ann. Physik (5) 26. 55 (1936). 



6. MoiiiRi', K., personal communication (1956). 



7. MoLiERE, K. and Niehrs. H.,Z. Physik 140, 581 (1955). 



8. Rees, A. L. G.andSpiNK. J. A.,/fc/fl C/-v.vr. 3, 316 (1950). 



9. RiECKE, W. D., P/n.s. It'///. (2) 6. 20 (1955). 



10. RinrKE,W. D. and Rlska, E., Z. h/.v.v. Mikroskop.{\951, 



in press). 



11. Rt_SKA. E. and Wni rr. O., Z. u/.v.v. Mikroskop. 62, 465 



(1956). 



