Presence of tri-iodide ions in iodine-intercalated ${\mathrm{IBi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$ superconductors

Stage-1 iodine-intercalated ${\mathrm{IBi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_8$ single crystals with ${\mathit{T}}_{\mathit{c}}$=78 K were studied by Raman microspectroscopy in comparison with undoped ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_8$. Evidence is shown for the existence of an asymmetrical ${\mathit{I}}_3^{\mathrm{-}}$ state and a weak interaction is revealed between this ion and the adjacent Bi-O layers. The phonon coupling that initially exists in the identical, close, and staggered Bi-O bonds along the c axis, disappears as a result of the iodine intercalation in the host network where the Bi-O bonds become aligned along this c axis.