Raman studies of the anionic sublattice vibrations in (C5H5NH)6Bi4Cl18

Low frequency Raman spectroscopic studies (5–400 cm−1) were carried out for single crystal samples of (C5H5NH)6Bi4Cl18 within the temperature range 37–285 K to look for possible spectral changes resulting from the phase transitions at 122 and 154 K. A detailed analysis of frequencies, band widths and intensities of the bands was made for the (<f>a(ba)c&ast;</f>) and (<f>a(bb)c&ast;</f>) spectra through the phase transitions to understand their nature. It was shown that the transitions are not accompanied by any mode softening, thus indicating their order–disorder nature. However, the ordering of the pyridinium cations close to 122 and 154 K exerts a strong influence on the (Bi4Cl186−) anion. [Copyright &y& Elsevier]