RAMAN INVESTIGATION OF YBA2-XLAXCU3O7 CERAMICS

We present Raman data for the series of high-temperature superconducting compounds YBa{sub 2{minus}{ital x}}La{sub {ital x}}Cu{sub 3}O{sub 7} with {ital 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 {ital A}{sub {ital g}} modes on {ital x}. For {ital x}=0.4 a structural phase transition from orthorhombic to tetragonal takes place. With decreasing orthorhombicity ({ital a}{minus}{ital b})/({ital a}+{ital 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 {minus}52.9 cm{sup {minus}1}. With increasing {ital 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 cm{sup {minus}1}. Both peaks are assigned to a phonon-density-of-states contribution to the Raman spectra induced by the Ba-La disorder. {copyright} {ital 1996 The American Physical Society.}