Ab initio lattice dynamics and thermal expansion of Cu2 O

Using ab initio density-functional perturbation theory the lattice dynamics of Cu2 O has been extensively studied. Discrepancies with older neutron-scattering results triggered a neutron-scattering investigation of our own which confirmed the theoretical prediction. Based on the accurate description of the phonon dispersion the vibrational part of the free energy was calculated as a function of lattice constant and temperature. The thermal expansion could be obtained in good agreement with experiments. For T<300 K negative thermal expansion (NTE) was observed while for higher temperatures normal thermal expansion was obtained. The origin of the NTE is due to anomalous behavior of phonon modes with energies <20 meV which is highly mode and wave-vector sensitive. Our calculations also showed that free energies based on the exact phonon dispersion and those based on an Einstein-model description (widely used) differed by more than 20%, which clearly indicates the importance of proper phonon-dispersion treatment for obtaining reliable thermodynamic data.