RAMAN-SPECTROSCOPY OF RARE-EARTH-SUBSTITUTED YBA2CU4O8

Ceramic samples of the superconductor RBa2Cu4O8, with R=Y, Er, Dy, Gd, and Sm have been prepared and studied by Raman spectroscopy in the ra nge of 180-700 cm-1. Most of the modes are found to shift more slowly with ionic radius than in the related compound RBa2Cu3O7-delta, althou gh the expansion of the lattice is similar in the two materials. The o ne exception is a 340-cm-1 mode associated with the oxygen ions on the CuO2 planes, which has a mode Gruneisen parameter of 3.3 at room temp erature. Raman spectra taken below room temperature show little change other than the narrowing expected from the increased lifetime as anha rmonic two-phonon decay becomes less important. The 340-cm-1 mode is t he exception; its frequency displays a temperature dependence that is especially strong in the sample with the largest rare-earth ion (Sm). Even in this case there is no change that can be related to the superc onducting transition, in contrast with observations in RBa2Cu3O7-delta . The absence of changes near T(c) is discussed in terms of the magnit ude of the gap and its dependence on the concentration of holes on the CuO2 planes.