The Jahn-Teller Effect in Solids

The Jahn–Teller effect is the intrinsic instability of an electronically degenerate complex against distortions that remove the degeneracy. Until recently, most of the effort in this field has been theoretical, and there has been more emphasis on mathematical sophistication than on the prediction of observable effects. In the past year or so, this situation has changed. Theoretical predictions have been made that can be tested rather directly against experiment. This chapter presents a general account of the basic theory and its possible applications. It applies this theory to the particular problem of a doubly degenerate electronic state interacting with a doubly degenerate vibrational mode. It extends the treatment to triply degenerate states and deals with the problem of electronic transitions involving degenerate states. In each section, as the theory is worked out and applied to particular cases, experimental results are quoted to illustrate how well (or how badly) the theory works out in practice. By this means, this chapter attempts to draw attention to gaps both in the experimental data and in theoretical understanding.