Infrared investigation of the order-disorder phase transitions in (C5H5NH)(6)Bi4Cl18

Infrared spectra (4000-500 cm(-1)) of polycrystalline samples of (C5H5NH)(6 )Bi4Cl18 were investigated in the temperature range 23-309 K. An assignment of the observed bands at 23 and 309 K is proposed. A temperature dependenc e study of the frequencies, nu, half-widths, Delta nu (1/2) and intensities of some internal bands of organic cations confirm the presence of phase tr ansitions at 122 and 154 K. In the low-temperature phases numerous internal modes show splitting that indicates a distinct change in the motional stat e of the pyridinium cations. The spectroscopic results are consistent with earlier H-1 NMR studies showing that the phase transitions are governed by the in-plane complex motion of the C5H5NH+ cations. (C) 2000 Elsevier Scien ce B.V. All rights reserved.