Spurious piezo and pyroelectricity of diacetylenes

Sample defects can be another origin of piezo and pyroelectric phenomena in substituted diacetylenes, which generally can be identified via sample dependence and smaller size of these effects [88]. Bloor et al. discussed such spurious pyroelectric effects, which seemed to be correlated with the occurrence of macroscopic deformations, such as screw dislocations in TS single crystals [90]. Similarly the analysis of weak polarization, caused by molecular distortion during solid state polymerization of TS, was reported by Bertault et al. [91]. We have observed comparable weak pyroelectric phenomena for TS/FBS [87]. 

Whereas several pyroelectric diacetylenes were identified, uniform ferroelectric phases seem to be rather the exception, according to our experience with many new substitutions of diacetylenes [92]. The ferroelectric low-temperature phase of the symmetrically disubstituted diacetylene DNP, i.e. l,6-bis(2,4-dinitrophenoxy)-2,4-hexadiyne (Tab. 9.2), turned out to be one of the rare and interesting exceptions [93-98]. 

The temperature dependence of the spontaneous polarization (Fig. 9.39) of the monomer crystal can be described in the framework of Landau s phenomenological theory assuming a tricritical phase transition [97]. The maximum experimental value of the electric polar- 

The electric permittivity (Fig. 9.40) reaches values of about Sr 150 at the transition temperature. Its temperature dependence is strongly influenced by defects [97]. Thus, it is less appropriate for a comparison with theoretical predictions. 

In the early days of TOPOMAK, H. Schultes has already observed that the phase transition of DNP shifts to lower temperatures (Fig. 9.41-43) with increasing polymer content 

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