  While most solids expand when heated, some materials show the opposite
behavior: negative thermal expansion (NTE). In polymers and biomolecules, NTE
originates from the entropic elasticity of an ideal, freely-jointed chain. The
origin of NTE in solids has been widely believed to be different. Our neutron
scattering study of a simple cubic NTE material, ScF3, overturns this
consensus. We observe that the correlation in the positions of the neighboring
fluorine atoms rapidly fades on warming, indicating an uncorrelated thermal
motion constrained by the rigid Sc-F bonds. This leads us to a quantitative
theory of NTE in terms of entropic elasticity of a floppy network crystal,
which is in remarkable agreement with experimental results. We thus reveal the
formidable universality of the NTE phenomenon in soft and hard matter.
