  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.
