Structure and phase transitions in guanidinium halogenobismuthates(III)

Differential scanning calorimetry (DSC), dilatometric and dielectric measurements have been used to study the phase transitions in [C(NH2)(3)](3)BiBr6. The [C(NH2)(3)](3)BiBr6 crystal undergoes four phase transitions: at 419, 429, 475 and 495 K (on heating). The high-temperature phase transition is clearly of first-order type and its high entropy effect allows classifying it as of the order-disorder type. X-ray diffraction. studies showed that [C(NH2)3]3BiBr6 crystallises in monoclinic, centrosymmetric space group, C2/c. The anionic sublattice is composed of isolated BiBr63- octahedra. The infrared between 100 and 350 K and Raman spectra at room temperature in the frequency range related to the internal vibrations of guanidinium cations were studied for [C(NH2)(3)](3)BiBr6, [C(NH2)(3)](3)Bi2Br9, [C(NH2)(3)](3)BiCl(6)xCl(2) and C(NH2)(3)Cl to analyse the dynamical state of cations. At room temperature, the cations in the studied guanidinium halogenobismuthates(III) are almost rigid, so the mechanism of high-temperature phase transitions is probably connected with the release of rotation of the cations. (C) 2001 Elsevier Science B.V. All rights reserved.