Structure and phase transitions in the ferroelastic [C(NH2)(3)](3)Bi2Br9 crystal

Differential scanning calorimetry, dilatometric, dielectric and linear bire fringence measurements have been used to study the ferroelastic [C(NH2)(3)] (3)Bi2Br9 crystal. The x-ray studies showed that it crystallizes at room te mperature in the monoclinic symmetry, space group P2(1)/m. The crystal unde rgoes a complex sequence of phase transitions: at 311 K, 333.5 K, 350 K, 41 5 K and 425 K. All phase transitions were found to be of first order type. The ferroelastic domain structure is maintained from room temperature up to 425 K. The temperature measurements of the linear birefringence and optica l observations suggest the tetragonal symmetry of the parent paraelastic ph ase above 425 K.