Abstract
The negative thermal expansion (NTE) mechanism in Cu2O has been characterised via mapping of different Cu2O structural flexibility models onto phonons obtained using ab-initio lattice dynamics. Low frequency acoustic modes that are responsible for the NTE in this material correspond to vibrations of rigid O-Cu-O rods. There is also some small contribution from higher frequency optic modes that correspond to rotations of rigid and near-rigid OCu4 tetrahedra as well as of near-rigid O-Cu-O rods. The primary NTE mode also drives a ferroelastic phase transition at high pressure; our calculations predict this to be to an orthorhombic structure with space group Pnnn.
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