Re-induced Raman active modes in HgBa2Can-1CunO2n+2+delta compounds

A comparative Raman study of Re-free and Re-doped HgBa2Can-1CunO2n+2+delta with n = 1,3,4,5 is presented in an attempt to further clarify the structur al 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 s amples and are attributed to the oxygen modes of a strongly bound, almost d ecoupled ReO6 octahedron. As regards the apex oxygen in the Hg site, a clea r transfer of spectral weight from the 590 to the 570 cm-L 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 proba bly attributed to c-axis vibrations of Ba whose frequency shows a distincti ve change with the number n of the CuO2 layers, providing an easy way to id entify the various phases in a sample. [S0163-1829(99)01626-4].