79
Values of h2+k2+l2 between these limiting K£ values will
define a plane (h,k,l) that will diffract in the FOLZ. As
an example, consider the solution to a CBED pattern from
the gamma prime phase of RSR 197. The approximate
constants for substitution into the equations are
aQ 3.36,
R = 27.07 (100 keV electrons),
B = <114>,
and alpha = 2.5 x 103 rads.
The calculated values of and are 101 and 82,
respectively. Any plane h, k, 1 with values of h2+k2+l2
between 82 and 101 which also satisfies the g*b = 2
criterion will diffract under Bragg conditions and will
thus yield a FOLZ line in the transmitted disc. Possible
values are given in Table 4.1.
The value of K2 for the zero convergence case is the
exact Bragg solution. It is seldom an integer. If its
value were an integer, a set of conditions could be
achieved that would yield some set of FOLZ lines directly
through the center of the pattern. For example, for the
case just calculated, h2+k2+l2 = 90.713. This value is
very close to the exact solution for h2+k2+l2 (h=9, k=3,
1=1) = 91.0. The (931) lines should pass almost directly
through the center of the transmitted disc. They could be
made to pass directly through the center by either
increasing the alloy lattice parameter from 3.560 to 3.571
Angstroms, or by changing the accelerating potential