The anharmonic behavior of the oxygen modes in YBa 2 Cu 3 O x compounds

The anharmonic behavior of the Raman‐active oxygen modes was examined in YBa 2 Cu 3 16,18 O x compounds over the whole range of oxygen doping (6 < x < 7). The apex phonon of A g symmetry shows a small anharmonicity in the range of 6.4 < x < 6.9, which can be attributed to the anharmonic behavior of this mode in one of the constituent phases (ortho‐II, ortho‐III, etc.), but not in the ortho‐I phase. The B 1 g mode does not show any anharmonicity over the whole range of oxygen doping, but characteristic spectral changes appear for x ≈ 6.3 and 6.7. At the lowest oxygen content studied ( x = 6.03), there is a considerable amount of anharmonicity for the in‐phase phonon of the plane oxygen atoms, which remains large until x ≈ 6.3, at the phase transition to the tetragonal phase, where it drops to zero. It increases again for x ≈ 6.4 and then gradually decreases with doping to disappear at the optimal doping. If these changes are combined with the spectral modifications of the B 1 g and in‐phase modes, they indicate that local structural changes are induced by the transfer of carriers to the CuO 2 planes at x ≈ 6.3, which are related to the reduction in anharmonicity, the pinning of the carriers and superconductivity. Copyright © 2001 John Wiley & Sons, Ltd.