Phase equilibria in metastable regime in the (C8H12NO)2[ZnCl4] ferroelectric system

Abstract

(C₈H₁₂NO)₂[ZnCl₄] is an interesting ferroelectric organic–inorganic hybrid compound that shows a complex structural phase diagram, investigated in this work by X-ray diffraction and differential scanning calorimetry, characterized by an irreversible transition occurring on heating at 390 K, and coupled with successive transformations taking place on cooling in a metastable regime. The basic structural transition, leading from a monoclinic P2₁ to an orthorhombic P2₁2₁2₁ symmetry and involving the doubling of the unit cell volume, is associated with an order–disorder transformation of the 4-methoxybenzylammonium cations coupled with a reorientation of tetrachloridozincate anions. During cooling, the cationic order and the monoclinic symmetry are progressively restored, but the doubling of the original cell volume is maintained by the alternate canting of adjacent cationic layers. It is noteworthy that all the polymorphs preserve the non-centric nature of the structure, but the ferroelectric behavior, characterized by high saturation polarization P_S = 17 μC cm⁻² and relatively low coercive field E_C = 8 kV cm⁻¹, is lost in the transition on heating and not recovered at low temperature when the original polar P2₁ symmetry is restored.