LATTICE-DYNAMICS OF YBA2CU3O7-X IN OXYGEN-VACANCY ORDERED ORTHO-II AND ORTHO-III SUPERSTRUCTURES

A lattice-dynamics of YBa2Cu3O7-x in superstructures ortho-II and orth o-III of oxygen ordering (x = 0.5 and x = 0.33) is calculated in the f rameworks of shell model with transferable interatomic potential param eters. Frequencies and eigenvectors for 75 vibration modes of ortho-II and 114 phonons of ortho-III are analysed and their connection with t he phonon dispersion in YBa2Cu3O7 and YBa2Cu3O6 is investigated. In sp ite of the great complication of phonon spectra due to a multiplicatio n of a unit cell in superstructures ortho-II and ortho-III a lot of th eir long-wave phonons are still close to the short-wave eigenmodes of ortho-I and are not expected to be observable in optical experiments. All Gamma-phonons of ortho-I lattice manifest themselves in the long-w ave spectra of high-order phases. The intrinsic Gamma-modes of apical oxygen of YBa2Cu3O6 arise in both superstructures as well. Only a rest ricted set of long-wave phonons can be regarded as the inherent ortho- II and ortho-III modes and could be useful as the reference vibrations for the purpose of superstructure characterization. (C) 1998 Elsevier Science B.V.