Point defects and relaxation phenomena in crystals

6. Summary

Group theoretical methods are developed for studying two types of point defects in crystals, namely trapped and untrapped ones, and the relaxation phenomena that arise therefrom. It is pointed out that a very close analogy exists between the relaxation modes on the one hand and normal oscillations on the other. This analogy enables us to utilise many of the concepts developed and the results obtained earlier in connection with studies on molecular spectra and Raman effect for an investigation of the relaxation phenomena. Simple rules are given for writing out the modes, ascertaining their activity or otherwise in regard to external electric and mechanical fields, evaluating the relaxation times and so on. Specific examples which are of practical interest are worked out under both the classes of defects studied. The methods developed can be applied to any general case.