In the case of thin TEM films, the relaxation of the Bragg conditions occurs at the reciprocal lattice points. Those reflection (diffraction) points are transformed into relrods elongated normal to the TEM film as shown in Figure 3909a. This is also the reason why reflection spots are always shown in the electron diffraction pattern (EDP) even though the "perfect" Bragg condition sometimes is not exactly satisfied. Since this elongation is inversely proportional to the specimen thickness (t), the relrods become very elongated for very thin TEM films so that many of them can simultaneously intersect the Ewald sphere and produce diffracted beams. The number of the shown diffraction spots on the TEM screen or detector is more than that from a thick TEM film. In this case, when the specimen or the electron beam is tilted, the spot position in the diffraction pattern moves because the Ewald sphere moves and then intersects the different portions of the relrods (associated with the reciprocal lattice) to produce the diffracted
Figure 3909a. Schematic illustration of relrods elongated normal to the TEM film. The points of the reciprocal lattice are transformed into elongated relrods. The inset on the top-right-hand side is a zoom-in of the gold-cycled part.
Figure 3909b shows the schematic illustration of the reflection- and forbidden-relrods of zone axis diffraction patterns at different specimen thickness. At zero tilt, the Ewald sphere is tangential to zero-order Laue zone (ZOLZ) of the reciprocal lattices in Figures 3909b (a), (b) and (d). For very thin specimens, the forbidden-relrods (D2 and D7) do not appear as shown in Figure 3909b (a), while they appear when the thickness of the specimens increases as shown in Figure 3909b (b) and (d) and the visible forbidden-relrods elongate more for thicker specimens. Note that the forbidden-relrods become visible due to double diffraction in thicker specimens. When the specimen is tilted, the loci of the intersections of Ewald sphere with the relrods does not change, and thus the projected positions of reflections do not move. However, the excitation errors (s) for the relrods change significantly, so that reflection intensities in the electron diffraction patterns change significantly. Especially, for the relatively thin specimen as shown in Figure 3909b (c), the forbidden-relrods (D2 and D7) will disappear on the recorded diffraction pattern (intersections of Ewald sphere with the relrods) because they are too short to appear with specimen-tilting. However, for very thick specimens as shown in 3909b (d), both the reflection- and forbidden-relrods are almost in the same length so that the forbidden-relrods will not disappear before reflection-relrods with specimen-tilting.
Figure 3909b. Schematic illustration of reflection- and forbidden-relrods of zero-order Laue zone (ZOLZ), depending on film thickness and specimen tilting: (a) At zero tilt, very thin TEM specimen for which forbidden-relrods do not exist; (b) At zero tilt and (c) at high-angle tilt, not very thin TEM specimen for which short forbidden-relrods appear; and (c) At zero tilt, very thick TEM specimen for which long forbidden-relrods appear. Here, t1 < t2 < t3.
Unlike diffraction patterns which blink on and off, the Kikuchi line patterns rotate when the crystal is tilted. Therefore, tilting TEM sample along Kikuchi lines is a convenient way to orientate a crystal to certain zone axis.