RAMAN MODES OF YBA2CU3OX WITH VARIABLE OXYGEN-CONTENT

We have measured the Raman spectra of single crystals of YBa2Cu3O7-delta as a function of oxygen concentration, that is Y123-Ox, where x varies from approximately 7 (superconducting) to 6 (insulating). We have made similar measurements on single crystals of Sm123, Gd123, and Ho123. The experimental results from all of these single crystals are very similar. Our single crystal results are in distinct disagreement with experimental results on ceramics of these same materials. For x = 7 the usual five, c-axis polarized modes are observed; mode #5 occurs at approximately 500 cm-1 and is primarily due to motion of oxygen atoms in the four-coordinated Cu chain. At oxygen contents near the metal-insulator (M-I) phase transition, the intensity of mode #5 rapidly decreases and disappears, being replaced by a strongly (zz) polarized mode at almost-equal-to 600 cm-1. We assign the 600 cm-1 mode to oxygen atom vibration in the two-coordinated Cu "sticks". At the oxygen content near the M-I phase transition, other weaker lower-energy modes appear and these are discussed. However, these modes disappear for oxygen content almost-equal-to 6, where the appropriate number of symmetry allowed modes (five) is again observed. We believe that our results explain the "drastic difference" in the neutron-measured phonon density of states between x = 6 and x = 7. Much of the difference in phonon frequencies is not due to a decrease in the force constants in going from x = 6, to x = 7, but is due to phonon differences resulting from the coordination changes of the chain-Cu atoms. Other effects that occur near the M-I phase transition are discussed. Only some of these effects are in agreement with the Ortho II model.