Abstract
The vibrational motion of Fe atoms in Bi2Sr2Ca1Cu1.96Fe0.04Oy studied by means of Mossbauer spectroscopy was found to be very anharmonic. This anharmonic behaviour can be explained by local structural excitations between two or more states supposedly existing in the material rather than by the usual phonon-phonon interactions. The increase in Tc for the annealed Bi2Sr2Ca1Cu1.96Fe0.04Oy is attributed to the increase in local structural instabilities which enhances the effective electron-electron pairing.