Structure and ferroelectric properties of (C₃N₂H₅)₅Bi₂Cl₁₁

Pentakis (imidazolium) undecachlorodibismuthate(III), (C₃N₂H₅)₅Bi₂Cl₁₁, has been synthesized and studied by means of the x-ray, differential scanning calorimetry (DSC), and dielectric methods over a wide temperature range. The room temperature crystal structure has been determined as monoclinic, space group P2₁∕n with a=13.616 Å, b=14.684 Å, c=9.045 Å, and β=96.85°. Two solid–solid phase transitions: P4̅ n2→360 KP2₁∕n (I→II), and P2₁∕n→166 KP2₁ (II→III), discontinuous and continuous in nature, respectively, have been revealed. The dielectric and pyroelectric measurements allow us to characterize the lowest temperature phase (III) as ferroelectric with the Curie point at 166 K and saturated spontaneous polarization value of the order of 6×10⁻³ C m⁻² along the b axis. The ferroelectric phase transition mechanism is governed by the dynamics of imidazolium cations. The room temperature, paraelectric phase (II) demonstrates additionally the ferroelastic properties.