exaggerated compared to the micrographs. The calculations

required to determine the line positions of the patterns

are summarized in Appendix A.

 Since only a relative change in lattice parameter can

be realistically measured, measuring this relative change

involves measuring and quantifying changes in the HOLZ

line positions from one pattern to the next. This is

easily and most accurately accomplished by using HOLZ

lines that both intersect at shallow angles, and most

desirably, that move in opposite directions to one another

when the lattice parameter is varied. Distances between

intersections are then measured and these distances

ratioed for different values of absolute lattice

parameter. Figure 4.10 is a plot of the ratio a/b versus

relative change in lattice parameter. Parameters a and b

are defined in Figure 4.9. The calculation of the values

for Figure 4.10 is given in Appendix A.

4.1.5 The Effect of Strain and Non-Cubicity on
 Pattern Symmetry

 If a previously cubic crystal is nonisotropically

strained or has become noncubic due to transformation or

change in order, the CBED pattern will reflect this change

by a reduction in the symmetry of the HOLZ line pattern.

The CBED technique is especially sensitive, theoretically

capable of detecting changes in lattice parameter on the

order of two parts in ten thousand at 100 kV (Steeds,