We present Raman data for the series of high-temperature superconducting compounds ${\mathrm{YBa}}_{2\mathrm{-}\mathit{x}}$${\mathrm{La}}_{\mathit{x}}$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ with x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5. We have investigated the dependence of frequencies and linewidths of the five Raman-active ${\mathit{A}}_{\mathit{g}}$ modes on x. For x=0.4 a structural phase transition from orthorhombic to tetragonal takes place. With decreasing orthorhombicity (a-b)/(a+b) the O(2)-O(3)-in-phase and O(2)-O(3)-out-of-phase modes increase their coupling, contrary to what one may expect. This can be understood in terms of a negative contribution of the chain fragments to the coupling matrix element between these two modes, which we estimate to be -52.9 ${\mathrm{cm}}^{\mathrm{-}1}$. With increasing x, two additional features become more pronounced in the Raman spectra. The first is a broad hump in the vicinity of the Ba peak and the second a peak at 570 ${\mathrm{cm}}^{\mathrm{-}1}$. Both peaks are assigned to a phonon-density-of-states contribution to the Raman spectra induced by the Ba-La disorder. © 1996 The American Physical Society.

• Received 12 September 1995

DOI:https://doi.org/10.1103/PhysRevB.53.3561