Figure 1
Chen W. Li et al., Phys. Rev. Lett. (2011)

Rocking to a new beat. The shrinking-on-heating property of scandium trifluoride is largely due to the pivoting, or rocking, of octahedral-shaped units (purple) in the atomic structure. Vibrations in crystals are usually governed by a potential that increases as the square of the atomic separation. But this motion in scandium trifluoride is governed by a previously unobserved quartic potential, which varies as the forth power of the separation.