The ferroelectric low-temperature phase of single crystals of the substituted diacetylene 1,6-bis(2,4-dinitrophenoxy)-2,4-hexadiyne (DNP)

Dielectric and pyroelectric properties of single crystals of 1,6-bis(2,4-dinitrophenoxy)-2,4-hexadiyne (DNP) were measured parallel to the polar b axis. Poling experiments with static electric field strengths of up to 33 kV cm^-1 prove that, at least for parts of the crystals, the direction of the spontaneous polarization can be reversed, implying a distribution of transition temperatures. The maximum polarization measured reveals values up to 2.4*10^-7 C cm^-2 at 10 K, which is in very good agreement with X-ray studies. Within a Landau theory of phase transitions for ferroelectric materials, the experimental curves of the spontaneous polarization seem to behave tricritically in the polar phase below 46 K. Owing to the distribution of transition temperatures, the order of the phase transition cannot be determined from permittivity experiments. The influence of a small static electric field on the electric susceptibility is discussed as well.