Vibrational spectra and lattice instabilities in the high-Tcsuperconductors YBa2Cu3O7and GdBa2Cu3O7

The exceptionally high $T_c$ of layered cuprates was proposed recently as originating from electronically driven structural instabilities. We have studied the infrared and Raman spectra of $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ and $\mathrm{Gd}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ over a broad range of temperatures, from 10 to 300 K. We observed neither mode softening nor any other spectroscopic signature of lattice instabilities.