Thermal, dielectric properties and vibrational spectra of [n-C4H9NH3]2[BiBr5] crystal

[n-C4H9NH3]2[BiBr5] exhibits five solid–solid structural phase transitions; at 309 (1st order), 291 (1st), 240 (1st), 219 (close to 2nd) and at 166 K (1st). The phase transitions at 291 and 240 K are accompanied by distinct dielectric anomalies. The dielectric relaxation process was found to appear over the phases (III) and (I). The infrared spectra for [n-C4H9NH3]2[BiBr5] were measured in the region of the internal vibration of the n-butylammonium cation (4000–400 cm−1) between 12 and 320 K. The temperature changes of wavenumbers and of a width for many of the internal modes of the n-butylammonium clearly indicate the contribution of the organic sublattice to the phase transition mechanism.