We present new Raman scattering data on high-temperature superconducting materials with composition $\mathrm{Y}{\mathrm{Ba}}_2{\left({\mathrm{Cu}}_{1-x}{\mathrm{Al}}_x\right)}_3{\mathrm{O}}_{7-\delta }$. In $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ we observe new high-frequency peaks greater than 700 ${\mathrm{cm}}^{-1\mathrm{}}$ that we associate with two-phonon interactions of the first-order Cu—O Raman vibrations that occur at 337, (450±10), and 502 ${\mathrm{cm}}^{-1\mathrm{}}$. The intensity of these new peaks suggests that anomalous phonon interactions are taking place. Results on Al substitution are consistent with a unit-cell change from orthorhombic to tetragonal. The observation of superconductivity in the tetragonal structure means that it is not possible to simply assign superconductivity to the orthorhombic structure.

• Received 28 August 1987

DOI:https://doi.org/10.1103/PhysRevB.36.8315