Abstract
Single crystals of pyridazine perchlorate[(C4H4N2)HClO4] have been synthesized and characterized by x-ray diffraction, dielectric measurementsand optical studies. At room temperature the crystal is monoclinic, space groupP 21/n. The temperature dependences of the electric permittivity and birefringence exhibitanomalous behaviour, characteristic of the first-order phase transitions at 343 and 339 K onheating and cooling, respectively. Optical investigations under a polarizing microscope showthat the crystal symmetry changes from the biaxial room temperature phase II to theoptically uniaxial high temperature phase I. The domain pattern arising after cooling fromphase I reflects observed symmetry changes at phase transition. The fully disorderedstructure of phase I can be described in the obverse hexagonal space group .
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