RAMAN INVESTIGATION OF I3- IONS IN I-INTERCALATED BI2SR2CAN-1CUNOX, N = 1, 2

We report polarisation-resolved Raman measurements on iodine intercalated single crystals of the n = 1 and 2 members of the bismuth cuprates, Bi2Sr2Can-1CunOx. The spectra clearly show scattering by the symmetric stretch mode of triiodide ions, I3-, and the polarisation dependence implies that the ions exist in the lattice with a consistent alignment along either the a- or the b-axis. These conclusions are at variance with TEM results, which have been interpreted as indicating that the iodine is found as single ions. The Raman results further imply that the ions are in sites of relatively low symmetry, and we suggest a suitable site between the facing BiO layers. In addition to the line spectra the material shows a strong background signal, similar to but much stronger than is shown by the non-intercalated material, and which is probably of electronic origin. There is coupling between the triiodide mode and the electronic excitations, as signalled by an asymmetry of the vibrational lines. We have followed variations of the I3- mode frequency, width and asymmetry with temperature in the 8-300 K range.