LATTICE-DYNAMICS OF YBA2CU3O6+X (X = 0.25, 0.5, 0.75)

The lattice dynamics of YBa2Cu3O6+x with partially filled O(4) sites is investigated using a supercell method. The model used is the lattice-dynamical shell model, which incorporates the short-range interactions and the long-range Coulomb potential as well as the polarisabilities of ions. Varying the oxygen content produces small changes throughout the spectrum of normal modes, as well as major changes in those modes with large O(4) amplitudes. Selected phonon dispersion curves are presented, with emphasis on the variation of frequency with oxygen concentration. The nature of new modes, which appear with an increase in oxygen concentration, is investigated. The overall phonon density of states is also presented.