Systematic Raman study of effects of rare-earth substitution on the lattice modes of high-Tcsuperconductors

We report on the effect of rare-earth substitution on the Raman spectra of $R{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$ perovskites where $R=\mathrm{Y},\mathrm{La},\mathrm{Pr},\mathrm{Nd},\mathrm{Sm},\mathrm{Eu},\mathrm{Gd},\mathrm{Dy},\mathrm{Ho},\mathrm{Tm},\mathrm{and} \mathrm{Lu}$. These spectra show both the presence of Raman modes that are intrinsic to the superconducting material and also those due to impurity phases. The intrinsic Raman modes near 340 ${\mathrm{cm}}^{-1\mathrm{}}$ and 500 ${\mathrm{cm}}^{-1\mathrm{}}$ which have been attributed, respectively, to Cu(2)-O(2,3) bending and Cu(1)-O(4) stretching vibrations, show substantial and systematic variations in vibrational frequency as a function of the ionic radius of the rare earth. These variations are in good agreement with neutron-scattering determinations of bond lengths, which supports the vibrational assignment of these modes. Pr${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ shows no anomalous behavior in its vibrational spectrum even though it was the only sample studied that was not superconducting. This suggests that the valency of Pr is similar to the other rare earths studied and cannot account for the absence of superconductivity in this material. Assignment of other Raman modes in the rare-earth spectra requires identification of impurity phases, which even in small concentrations, can significantly contribute to the Raman spectra. Impurity phases can be preferentially at the surface of samples and can be substantially modified by heat treatment in an argon atmosphere. These impurity phases have previously been confused with disorder-induced Raman modes.