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]₃BiBr₆. The [C(NH₂)₃]₃BiBr₆ 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]₃BiBr₆, [C(NH₂)₃]₃Bi₂Br₉, [C(NH₂)₃]₃BiCl₆xCl₂ and C(NH₂)₃Cl 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.