Pyroelectric properties for single crystals of the disubstituted diacetylene DNP

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Abstract
Dielectric and pyroelectric properties of single crystals of 1,6-bis(2,4 dinitrophenoxy)-2,4-hexadiyne (DNP) were measured for different crystal orientations and as a function of solid state polymerization. In monomer crystals at Tc = (46.4 ± 0.8)K an anomaly of the dielectric constant was observed with ε(Tc)/ε(293 K) = 18 ± 3. Below Tc spontaneous electric polarization appears with values up to 1.3 · 10-7Ccm-2 at 10 K. Orientation dependence of these anomalies indicates that point symmetry must be reduced from 2/m of the monoclinic nonpolar high temperature phase to 2, if the low temperature phase is a uniform one. Details of the phase transition and dependence of the electric properties on sample quality were analysed. With increasing amount of polymerization of DNP, the transition temperature Tc is lowered and the size of the dielectric anomaly and of the low temperature electric polarization decreases.