Disordered stacking arrangement in Ni(CN)2. Weak interactions between square-grid layers allow large amplitude vibrations that cause the layer dimensions to shrink with increasing temperature.
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Materials with low-dimensional structures (e.g. 2-D or layered materials such as graphene, high-temperature superconductors) are important precisely because low dimensionality often leads to unusual and useful physical properties. Nickel cyanide, Ni(CN)2, is a material with a layer-like structure that shows the unusual phenomenon of negative thermal expansion.
This means that the dimensions of its square-grid-like layers decrease with increasing temperature, in contrast to most materials which expand as the temperature is raised. A combination of neutron scattering and x-ray diffraction has been used to show that the crystal structure of this material actually exhibits long-range structural order in only two dimensions, with no true periodicity perpendicular to its grid-like layers. This lack of periodicity reflects the weakness of the interactions between layers. Because of this, the square-grid layers can vibrate very easily and essentially independently of one another, and this in turn gives rise to its negative thermal expansion behaviour.
AL Goodwin, MT Dove (Cambridge University), SJ Hibble, AM Chippindale, AH Pohl (Reading University), AC Hannon (ISIS)
Contact: Dr AL Goodwin, firstname.lastname@example.org
Further reading: AL Goodwin et al. Phys. Rev. B (submitted)
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