Structural Phase Transition in Pyridazine Fluoroborate Crystal

Single crystals of pyridazine fluoroborate [(C4H4N2)HBF4] have been grown and characterized by X-ray diffraction, DSC, dielectric measurements and optical studies. At room temperature the crystal is monoclinic, space group P2(1)/n. DSC measurement revealed the reversible phase transition at 333 and 327 K on heating and cooling, respectively. The temperature dependences of the electric permittivity exhibit anomalies characteristic for the first-order phase transitions at practically the same temperatures as those observed in DSC. Optical observations by means of polarizing microscope show that the crystal symmetry changes from the biaxial room temperature phase II to the optically uniaxial high temperature phase I. The orientation states domain arising after cooling from phase I reflects observed symmetry changes between monoclinic and hexagonal system at the phase transition. The crystal seems to be similar to pyridazine perchlorate [(C4H4N2)HClO4].