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

Differential scanning calorimetry, dilatometric, dielectric and linear birefringence measurements have been used to study the ferroelastic [C(NH₂)₃]₃Bi₂Br₉ crystal. The x-ray studies showed that it crystallizes at room temperature in the monoclinic symmetry, space group P2₁/m. The crystal undergoes a complex sequence of phase transitions: at 311 K, 333.5 K, 350 K, 415 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 optical observations suggest the tetragonal symmetry of the parent paraelastic phase above 425 K.