Experimental evidence for a continuous phase transition in a multidimensional ferroelectric

Two phase transitions in ferroelectric crystals of pyridinium tetrafluoroborate [C₅H₆N]⁺BF₄^- have been characterized by x-ray- and neutron-diffraction studies and measurements of permittivity, pyroeffect, and spontaneous polarization. At T₁=238.7K the crystals transform from R3m to $C2$ symmetry, and at T₂=204K to space group $P2.$ The ions are orientationally disordered in the prototype phase, and the ferroelectricity below T₁ is induced by the onset of ordering of the ions. The spontaneous polarization of 1.5μC/cm² was found from pyroelectric-effect measurements. The anisotropy measurements of permittivity indicate clearly that the phase transition at T₁ is continuous, which is the unique case corroborating Landau's approach of deducing the character of phase transitions from symmetry considerations for multidimensional ferroelectrics.