An example of a tiny Jahn-Teller energy producing a large Jahn-Teller effect

In its most dramatic manifestation the Jahn-Teller (JT) effect couples vibrations to orbital electrons to produce a less symmetric molecular or crystal structure than expected from pure electronic considerations, in what is usually called a Jahn-Teller distortion. However, the vibronic coupling does not need to be so large to produce large effects in optical spectra. In the present paper we use two independent numerical methods (Born-Oppenheimer and Lanczos) to calculate the low-temperature infrared emission spectra of Fe²⁺ as substitutiional impurity in semiconductors with zinc-blende structure.