Framework Flexibility and the Negative Thermal Expansion Mechanism of   Copper(I) Oxide, Cu2O
  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. 
Leila H. N. Rimmer, Martin T. Dove, Bj\"orn Winkler, Dan J. Wilson,   Keith Refson and Andrew L. Goodwin
cond-mat.mtrl-sci