  A systematic Raman study in the visible carried out on the YBa2Cu316,18Ox
(x=6-7) compounds, with isotopic substitution of 18O for 16O, has detected a
doping dependent deviation from harmonic behavior for the frequency shift of
the in-phase mode, a smaller amount of anharmonicity for the apex mode, and
almost no effect for the out-of-phase B1g-symmetry phonon. It appears that the
amount of anharmonicity depends strongly on the oxygen concentration; it
diminishes close to the tetragonal to orthorhombic structural phase transition
and close to optimal doping, while it reaches its maximum value for the
ortho-II and a tetragonal phase. The almost zero anharmonicity at optimal
doping persists even at 77K. The data in the overdoped oxygen concentration,
where a softening of the in-phase phonon frequency occurs, indicate that the
anharmonicity is not enhanced by the sudden increase in the CuO2 buckling. The
results fully agree with recent studies of the ortho-II phase but they do not
comply with a static double-well potential of the apical oxygen atom at optimal
doping.
