Incommensurately modulated structure of phenazine-chloranil acid at T = 139 K
What is it about?
The co-crystal of phenazine (Phz) and chloranilic acid (H2ca) undergoes several phase transitions upon cooling. At T = 139 K an incommensurate ferroelectric phase is present with a symmetry given by the superspace group P21(½ [sigma]2 ½)0 and [sigma]2 = 0.5139. The modulation mainly affects the positions of the protons within half of the intermolecular hydrogen bonds that are responsible for the spontaneous polarization in all three low-temperature phases.
Why is it important?
Evidence for proton transfer in part of the hydrogen bonds is obtained from the correlated dependence on the phase of the modulation of the lengths of bonds involved in resonance stabilization of the acidic anion. Incommensurability is explained as competition between proton transfer favored for single hydrogen bonds on the basis of pKa values and avoiding unfavorable Coulomb repulsion within the lattice of the resulting ionic molecules.