projected from the higher order zone onto the zero order

zone and are different from the calculated ones. The

difference is explained in Appendix B.

 Once the FOLZ has been indexed, FOLZ line positions

in the transmitted disc can be used to determine relative

lattice parameter differences and can be used to measure

small symmetry differences due to crystallographic changes

as a result of alloying, strain, or transformation.


4.1.4 Lattice Parameter Changes


 The easiest way to visualize the changes in the FOLZ

line position in the central spot that accompany changes

in lattice parameter is to look at a few examples of

patterns to see what happens when the lattice parameter is

varied. Figure 4.9a illustrates a case where the lattice

parameter of a diffracting crystal is such that at 100 kV,

the four 931 lines in a B = (114) pattern pass exactly

through the center of the transmitted disc. This is

equivalent to saying that the Bragg condition is satisfied

for g = (931) when B = (114). For this to be true, the

lattice parameter of the crystal must be exactly 3.5712

Angstroms. If the lattice parameter is greater than this,

a pattern like that shown in Figure 4.9b will result. If

the lattice parameter is less than 3.5712 Angstroms, the

pattern will look like Figure 4.8c. The shaded area in

the figure outlines the symmetry changes. This change is