EFFECT OF THE RARE-EARTH SUBSTITUTION ON THE OPTICAL PHONONS OF LARBACUO(5) (R=ND AND EU) OXIDES

In this work we present, on the one hand, the effect in the Raman spec tra is produced by the partial substitution of lanthanum by Nd or Eu i n LaRBaCuO(5). Because the atomic masses and ionic radii of these ions are quite similar the frequencies of the phonons principally related to them are not expected to vary significantly. In fact only small cha nges are observed in this direction. Nevertheless, substantial changes are observed in the intensities of the low frequency modes. The frequ encies of the oxygen vibrations are also influenced by the distortion produced by the rare earth substitution. On the other hand, we report the IR reflectivity spectra of polycrystalline samples of pure and mix ed compounds. The lowest frequency modes present small damping which p ermits us to observe variations in their frequencies corresponding to the different compounds. The effect of the quality of the pressed powd er on the IR reflectivity spectra is discussed.