and 5) the measurement of uniform lattice strain (Steeds,

1979).


4.1.1 Experimental Technique


 There are a number of methods for forming the

convergent probe. The most common method in modern STEM

instruments is to use the STEM "spot" mode to translate

the already convergent probe to the area from which the

pattern will be taken. In the STEM mode, the imaging and

projector lenses are already configured to form a

diffraction pattern. Convergence in the probe is

controlled by the selection of a suitable second condenser

aperture. Suitable is defined as the maximum aperture

size that will still give discreet, non-overlapping discs

in the diffraction patterns. The covergence angle alpha

is defined as the angle subtended by the radius of the

disc. Choice of the proper aperture size will obviously

depend on the lattice parameter and orientation of the

crystal from which the pattern will be taken.

 Even more simply, the probe may be focused directly

 in the TEM mode, a situation which yields acceptable beam

 convergence but usually with large probe sizes. Once the

 probe has been focused using the condenser controls, the

 proper lens excitations to image the diffraction pattern

 are selected, usually by selecting the diffraction mode of

 the instrument, and a CBED pattern results.