Phonon softenings and the charge density wave instability in R2O2Sb (R = rare-earth element)

To explore the origin of the insulating property of layered system R2O2Sb (R = rare-earth element) and to find their distorted superstructures, we have investigated the band structures and phonon dispersions of La2O2Sb and Er2O2Sb based on the ab initio density functional theory. We have found three dominant Fermi surface nesting vectors of q(1) = 0.25a*, q(2) = 0.5a* + 0.25b*, and q(3) = 0.25a* + 0.25b*. Phonon softening instabilities occur at the same q vectors, which suggests that the lattice instability in Sb layers is driven by the charge density wave instability through the electron-phonon coupling. The linearly superposed mode (eta(q3)*) of two degenerate normal modes at q3 produces the most stable state with a herringbone-type arrangement of Sb dimers. In the ground state, the distorted Sb square sheet shows the insulating behavior in agreement with existing experiments.