Raman measurements of YBa₂Cu₃O_x as a function of oxygen content

Abstract

We report polarized Raman measurements on single crystals of YBa2Cu3Ox, Y123-Ox, as a function of oxygen content. For x=7 the usual five, c-axis polarized modes are observed; mode #5 occurs ∼500 cm-1 and is due to the oxygen atoms in the four-coordinated Cu chain. At oxygen contents near the metal-insulator (M-l) phase transition the intensity of mode #5 rapidly decreases and disappears, being replaced by a strongly (zz) polarized mode at ≈600 cm-1. Our results are in sharp disagreement with published Raman results on ceramics. We believe the 600 cm-1 mode is due to oxygen atom vibration in the two-fold coordinated Cu "sticks". At the oxygen content near the M-l phase transition, other lower energy modes appear but they disappear for oxygen content x≈6 where the appropriate number (five) of symmetry allowed modes are again observed. A "drastic difference" in the phonon density of states between x=6 and x=7 has been obtained from inelastic neutron measurements. We believe that 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. © 1991.