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