A first-principles study of vibrational modes in Cu2O and Ag2O crystals

A first-principles investigation of cuprite crystals (Cu2O and Ag2O) has been performed. For Cu2O, the calculated frequencies at the Γ point of the Brillouin zone are in very good agreement with the experimental frequencies. For Ag2O, the presence of Eu and F2u vibrational modes with negative frequencies indicates a low temperature phase transition, in agreement with recent high resolution X-ray and neutron diffraction measurements. The energy scanning along these two modes shows a double-well potential, within which only the Ag atoms vibrate. As a result, the origin of the phase transition can be attributed to displacive disorder of the Ag atoms.