A more symmetrical position for an interstitial atom may be obtained if two atoms share the space of one.
This is illustrated below for a fcc crystal; a configuration like this is called a dumbbell. In the dumbbell configuration, the interstitial now has a preferred direction; it is no longer isotropic.
In bcc crystals, a dumbbell configuration exists, too; it is shown below. Again, the interstitial now has a preferred direction.

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© H. Föll (Defects - Script)