Framework flexibility and the negative thermal expansion mechanism of copper(I) oxide Cu2O

The negative thermal expansion (NTE) mechanism in Cu₂O has been characterized 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 an orthorhombic structure with space group Pnnn.