Ferroelectricity of Phenazine–Chloranilic Acid at $$T = 100$$ K

The co-crystal of phenazine (Phz) and chloroanilic acid $$(\hbox {H}_{2}\hbox {ca})$$ is ferroelectric below the temperature $$T_{c}^I = 253$$  K (FE-I phase). Upon cooling, two more phase transitions involve a further reduction of symmetry, until Phz-H $$_{2}$$ ca is triclinic in the second ferroelectric phase (FE-II phase) stable below $$T_{c}^{II} =$$ 137 K. Ferroelectricity in all low-temperature phases is believed to be related to partial proton transfer within the hydrogen bonds between the molecules Phz and $$\hbox {H}_{2}\hbox {ca}$$ . Here we present the crystal structure of the FE-II phase at $$T = 100$$  K. Experimental positions of hydrogen atoms indicate that ferroelectricity is mainly governed by half of the hydrogen-bonded chains, whereby proton transfer is observed within one of the two hydrogen bonds in which each molecule participates. A simple point charge model quantitatively reproduces the polarisation of this material. However, a possible contribution to the polarisation is proposed of the O–H $$\cdots$$ N hydrogen bonds of the second half of the mixed chains, which show elongated O–H bonds similar to those in the FE-I phase. The twofold superstructure with $$P1$$ symmetry was successfully solved as commensurately modulated structure employing the monoclinic superspace group $$P2_{1}(1/2\,\sigma _{2}\, 1/2)0$$ . The latter shows that the distortions at low temperatures follow a single normal mode of the space group $$P2_{1}$$ of the FE-I phase, and it thus explains that the direction of the polarisation remains close to the monoclinic axis, despite the lowering towards triclinic symmetry. Ferroelectricity below Tc(II) = 137 K is governed by proton transfer from O to N atoms within one quarter of the O–H⋯N hydrogen bonds, together with elongation of the O–H bonds within a second quarter of hydrogen bonds.