Re-induced Raman active modes in ${\mathrm{HgBa}}_2{\mathrm{Ca}}_{n-1}{\mathrm{Cu}}_n{\mathrm{O}}_{2n+2+\delta }$ compounds

A comparative Raman study of Re-free and Re-doped ${\mathrm{HgBa}}_2{\mathrm{Ca}}_{n-1}{\mathrm{Cu}}_n{\mathrm{O}}_{2n+2+\delta }$ with $n=1,3,4,5\mathrm{}$ 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 ${\mathrm{ReO}}_6$ octahedron. As regards the apex oxygen in the Hg site, a clear transfer of spectral weight from the 590 to the 570 ${\mathrm{cm}}^{\mathrm{-}1}$ 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 ${\mathrm{CuO}}_2$ layers, providing an easy way to identify the various phases in a sample.