Doping effects in the Sr14Cu24O41-type structure: A Raman scattering study

Polarized micro-Raman spectra of different compounds belonging to the Sr14Cu24O41-type structure were studied. In the spectra with parallel polarization along the plane crystal axes of the insulating samples Sr9R5Cu24O41 (R = La, Y) a broad peak near 3000 cm(-1), similar to the well-known two-magnon peak in the layered cuprates, was observed. In addition to the Raman lines characteristic for this structure, we observed only in the spectra with polarization parallel to the chains numerous lines between 100 and 1200 cm(-1), probably originating from Raman-forbidden infrared-active only LO phonons and their combinations. In the spectra of the conducting compounds Sr14-xCaxCu24O41 (x = 0, 7) these features were very weak or disappeared. We interpret these results as Raman evidence for hole doping of the Cu2O3 ''spin-ladder'' planes and for a redistribution of holes between chains and planes through Ca substitution in the case of the rare-earth-free samples. (C) 1997 Elsevier Science B.V.