Raman spectroscopy, structural modifications, and phase transitions in the high-temperature superconductors

The formation of phases is investigated from a systematic study of the Raman active phonons in the whole range of oxygen doping of YBa2Cu3Ox (6 < x < 7). Changes in the spectral characteristics of the in-phase vibrations of the plane oxygen atoms in the overdoped region are related, with slight modifications of the buckling in the CuO2 planes. A softening of the A(g) symmetry apical oxygen phonon is accompanied with an increase in the intensity of the high-frequency mode at similar to 600 cm(-1) and attributed to the breaking of the chains into small segments. The analysis of the apex and the in-phase phonon modes indicates the formation of superstructures, which are embedded as spatially separated phases in the whole oxygen concentration region of 6.35 < x < 6,95. A similar separation into small domains of phases may also occur in the underdoped compounds (x < 6.35) down to a critical oxygen concentration at x <similar to> 6.15, where the system breaks into randomly oriented small chains.