Raman Spectroscopy of Local Structure and Reordering Processes in YBa2Cu3O7-δ-Type Compounds

Abstract

The real structure of RBa2Cu3O7-δ (R=Y, rare earth) at intermediate values of δ (0<δ<1) is characterized by an inhomogeneous distribution of oxygen atoms in the basal (Cu1—O1) planes. At a microscopic level the sample can be considered as a multi-phase system, the relative weight of co-existing stable and unstable microphases being determined by the value of δ and the history of the sample. Standard x-ray and neutron diffraction techniques fail in distinguishing co-existing microphases as the typical size of corresponding ordered domains (10–100 lattice constants) is below the detection limit of these techniques. In contrast, the phonon frequencies (in particular of the Raman-active modes involving vibrations of mainly one type of atoms) are sensitive to the local atomic-site coordination which may differ significantly for the different microphases. Thus, Raman spectroscopy provides an opportunity to analyse the microphases in RBa2Cu3O7-δ and their mutual transformations with δ and/or under different treatments.