JahnTeller

The Jahn-Teller effect, sometimes written as Jahn–Teller or Jahn-Teller, is a geometric distortion of non-linear molecular systems that reduces electronic degeneracy and lowers overall energy. It was proposed by Hermann Arthur Jahn and Edward Teller in 1937. The Jahn-Teller theorem states that any non-linear molecule in an electronically degenerate state is unstable and will distort to lift the degeneracy, stabilizing the system.

In coordination chemistry and solid-state physics, the effect explains distortions in octahedral and other coordination environments of transition metals, as well as in various crystals. Typical cases involve partially filled degenerate electronic orbitals, such as the eg set in octahedral d9 ions (for example Cu2+) or in certain d4 configurations. Distortions often involve elongation or compression along a principal axis to lift degeneracy and lower the electronic energy. This can lead to changes in bond lengths, local symmetry, and spectroscopic properties.

The Jahn-Teller effect can be static, with a permanent distortion at all temperatures, or dynamic, where the molecule rapidly interconverts among equivalent distorted forms. In solids, dynamic distortions can average to higher apparent symmetry at room temperature, influencing physical properties such as conductivity and magnetism.

The phenomenon is described as vibronic coupling between electronic states and vibrational modes. In high-symmetry environments, electronic states of degeneracy (often labeled by E or T representations) couple to vibrational modes of compatible symmetry, driving distortions that remove degeneracy. Related concepts include the pseudo-Jahn-Teller effect, where non-degenerate states interact with excited states to produce distortion. Materials showing Jahn-Teller activity include certain copper oxides, manganites, and fullerene compounds.