Upper Laue zones

By definition, a zone axis is normal to both g and h and other reciprocal lattice vectors in the plane defined by these two vectors. The reciprocal lattice plane passing through the reciprocal lattice origin is called the zero-order zone axis. A G-vector with z - G=n with n O is said to belong to a high order Laue zones, which separate to upper Laue zones (n>0) and lower Laue zones (n<0). 

Fig. 40. (b) High magnification image of the [lOT] zone axis pattern related to the diffraction in (a). The lack of symmetry about an axis normal to the image is due to the influence of the image of the nodes of the upper Laue zones. Arrows indicate the position of the bend contours due to these nodes. 

Fig. 41. Intersection of the Ewald sphere with the reciprocal lattice of B, when the Ewald sphere is tangent to the (lOl) plane. This situation occurs for the electron beam parallel to the [101] axis. The [201] direction is slightly different from the direction of the radius of the Ewald sphere. Nodes of the upper Laue zones give extra spots in the [202] direction, but not near the origin of the reciprocal lattice. The asymmetry of the upper Laue zones is generally visible in electron diffraction.

Fig. 53. (a) General feature of the fine structure near a <111) zone axis pattern. This fine structure is due to the influence of the upper Laue zones, the image has only a three-fold symmetry, (b) Relation between two fine structures occurring on each side of a twin of C. 

Fig. 54. A small variation of the high voltage value modifies the intersection of the Ewald sphere with the plane of the upper Laue zone, so that the fine structure (due to the nodes of the upper Laue zones) is strongly modified by variations of the high voltage value.

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