Practical Examples of Point and Space Group Determination in Convergent Beam Diffraction

Abstract

The high quality of diffraction information generated by convergent beam electron microscopy provides many ways of arriving at an unambiguous assignment of a space group to a crystal under investigation. One method of space group determination involves tilting the crystal to a number of different zone axes and combining the information derived from each. However, it is often possible to obtain the same information more efficiently by concentrating on just one high symmetry zone axis and it is this approach which will be illustrated here. The prerequisites for the application of the technique are a good crystal and a microscope which offers a large angular view of the back focal plane of the objective lens. Specimen cooling is generally advantageous but has not been used in most of our work and, in particular, was not used for the results used as illustrations here. By the term 'good' crystal is meant a homogeneous and strain-free crystal without planar disorder (stacking faults, antiphase boundaries) linear disorder (dislocations) or point disorder. The requirements are certainly stringent but they need only operate over a cube of side approximately 1OO nm. Point disorder which produces an effect of the same order of magnitude as thermal disorder can be tolerated and strains less than 10-4are not detected. An angular view of 20° or so in the diffraction plane is generally acceptable although a larger view would be helpful for certain crystal structures and for low-specimen temperatures.