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. © 2014 American Physical Society