Entropic elasticity and negative thermal expansion in a simple cubic crystal

Abstract

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.

arXiv comment6 pages, 4 figures. Work reported at 2019 APS March meeting (http://meetings.aps.org/Meeting/MAR19/Session/B33.8)

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• Empty perovskites as Coulomb floppy networks: entropic elasticity and negative thermal expansion
• Mixed anion control of enhanced negative thermal expansion in the oxysulfide of PbTiO3
• Fundamentals on Dependence of Volume on Pressure and Temperature
• Inverse Melting of Polar Order in Chemically Substituted BaTiO3
• Comment on "Colossal Pressure-Induced Softening in Scandium Fluoride"