Jahn–Teller effect

Why do some crystals look distorted?

Imagine you're looking at a perfectly symmetrical crystal, but one side appears squished compared to the others. You can't figure out why it's not perfectly balanced.

The crystal isn't happy with its symmetry. It distorts itself to relieve some internal stress, much like you might stretch a rubber band to relieve tension. This is the Jahn-Teller effect.

Example

In an octahedral crystal, the copper ion distorts the surrounding ligands into a square planar shape because it's more comfortable there.

Remember this

The crystal distorts to lower its energy and relieve stress.

Text adapted from Wikipedia, licensed under CC BY-SA 4.0.

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