Optical methods as an aid in structure determination

Abstract

INTRODUCTION Optical diffraction analogues were first suggested in 1939 by Sir Lawrence Bragg' and, since then, many workers have used them in a great variety of ways. Although they have been used as an aid in single-crystal structure determination, their development to the required precision for use in serious research was a little too late; digital computers became widely accessible and made possible somewhat more objective and less intuitive methods. It is interesting to note, however, that in some developing countries where computers are not so readily available, optical methods provide a useful and relatively cheap alternative route to the solution of the basic phase problem. The most useful feature of optical methods for single-crystal work, however, remains the ease and rapidity with which ideas can be tried out, and hence they provide a rapid approach in teaching x-ray diffraction; their visual character makes it easy to achieve rapidly a familiarity with relationships between structures in real space and their diffraction patterns in reciprocal space. This, after all, is the central problem in all diffraction studies. A very brief summary only of the single-crystal aspect will be given. The advent of the gas phase laser has made it possible to rethink the basic design of optical diffractometers and a description of a current instrument will be given, together with new ideas for mask-making procedures and techniques. By far the most useful applications of optical methods, however, are in studies of imperfectly crystallized material of various kinds and the majority of the paper will be devoted to this aspect. Again, one of the most useful benefits of the optical approach is that ideas are generated which may enable one to break loose from preconceived notions that often inhibit progress. The method is not objective and does not provide a certain route to a solution; it is however, a valuable aid to intuition and in some problems provides the only practicable means of testing ideas.