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 T-1 = 238.7 K the crystals transform from R(3) over barm$ to C2 symmetry, and at T-2 = 204 K to space group P2. The ions are orientationally disordered in the prototype phase, and the ferroelectricity below T-1 is induced by the onset of ordering of the ions. The spontaneous polarization of 1.5 mu C/cm(2) was found from pyroelectric-effect measurements. The anisotropy. measurements of permittivity indicate clearly that the phase transition at T-1 is continuous, which is the unique case corroborating Landau's approach of deducing the character of phase transitions from symmetry considerations for multidimensional ferroelectrics.