Negative thermal expansion and local dynamics

A systematic EXAFS study has been performed on crystals affected by negative thermal expansion (NTE) and characterized by different structures: diamond-zincblende, cuprite, delafossite. Some general common features emerge from the results. The nearestneighbours bond thermal expansion measured by EXAFS is always positive; within each family of isostructural compounds, the stronger is the lattice NTE, the larger is the positive bond expansion. A correlation can be established between the NTE properties and the anisotropy of relative vibrations with respect to the bond direction. EXAFS results are consistent with phenomenological models of NTE based on the competition between stretching and tension effects.