Apex anharmonicity observed by Raman scattering in ¹⁸O substituted YBa₂Cu₃O_6+x

IR excited (1.16 eV) Raman spectra of several YBa2Cu3O6+x metallic samples (0.48<x<0.94) with both 16O and 96% substituted 18O have been performed. The dominant features of the spectra are two phonon bands around 340 and 490 cm-1 in 16O samples. In comparing the energy shift of these two Raman bands upon isotopic substitution with the calculated ones, the "harmonic" approximation accounts well for the shift of the lower of the two, the O(2)/O(3) "dimpling" vibration, whereas the higher O(4) band behavior is consistent with the existence of a highly anharmonic potential along the c-axis, a property otherwise only found in ferroelectrics. © 1994.