Re-induced raman active modes in HgBa₂Ca_n-1Cu_nO_2n+2+δ compounds

A comparative Raman study of Re-free and Re-doped HgBa₂Ca_n-1Cu_nO_2n+2+δ with n = 1,3,4,5 is presented in an attempt to further clarify the structural and phononic modifications brought about by Re substitution. A number of extra high-frequency phonon peaks show up in the spectra of the Re-doped samples and are attributed to the oxygen modes of a strongly bound, almost decoupled ReO₆ octahedron. As regards the apex oxygen in the Hg site, a clear transfer of spectral weight from the 590 to the 570 cm^{− 1} apex phonon band is observed upon Re substitution. Such a change may well be accounted for assuming increased excess oxygen content for the Re-doped samples. Another interesting result is the enhancement upon Re doping of a narrow peak probably attributed to c-axis vibrations of Ba whose frequency shows a distinctive change with the number n of the CuO₂ layers, providing an easy way to identify the various phases in a sample.