Isotope effect in anharmonic double-well potentials

Analysis of EXAFS in YBa2Cu3O7 shows that the axial oxygen O(4), associated with the Cu-O chains, moves in a double-well potential. Optical measuremen ts of the isotope effect in the frequencies of phonons involved in the axia l oxygen motion, show an isotope shift smaller than that obtained assuming a harmonic potential. In this work we present an isotope effect study of th e excitation frequencies of two anharmonic potentials, the phi(4) double-we ll and the Quadratic double-well (QDW) potentials, when the isotopic mass o f the axial oxygen is varied. The potential parameters used correspond to Y Ba2Cu3O7. We found that both anharmonic effective potentials phi(4) and QDW are unable to reproduce the experimental values of the isotopic coefficien t (frequency shift), suggesting the presence of a different kind of anharmo nicity.