The term might sound like a euphemism for misfortune, as in 'negative equity' or 'negative patient-care outcome', but materials that exhibit negative thermal expansion (NTE) are interesting and sometimes useful exceptions to the general rule that substances expand when heated. Negative or zero thermal expansion arises from electronic effects in some metal alloys, such as the iron–nickel alloy Invar (Fe0.64Ni0.36). Many materials in a second 'structural' class, in which NTE is associated with vibrations that bend an atomic network, have been discovered in the past 20 years. Deviations of the materials' lattice vibrations from the simplest (harmonic) behaviour are expected to contribute to NTE, but most of these materials have structures that are too complex for detailed vibrational analysis. Writing in Physical Review Letters, Li et al.1 provide an elegant demonstration of a non-harmonic (anharmonic) twisting vibration of a simple structural NTE material, scandium trifluoride (ScF3), that helps to explain the origin of NTE in this class.
Scandium trifluoride contains a simple network of corner-sharing ScF6 octahedra (Fig. 1). Other fluorides and oxides with the same structural arrangement do not show significant negative expansion, so it came as a surprise when a large NTE effect was discovered2 in ScF3 across a wide range of temperatures, from 10 to 1,100 kelvin. Previously reported materials with a comparable NTE, such as zirconium tungstate (ZrW2O8), have more complex crystal structures3, so ScF3 has proved an ideal material in which to explore the subtle features of lattice vibrations (phonons) that can give rise to NTE.
- NEGATIVE THERMAL-EXPANSION