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Free keywords:
protron transfer; ferroelectric
materials; incommensurate structure
Abstract:
The co-crystal of phenazine (Phz) and chloranilic acid (H2ca) becomes
ferroelectric upon cooling through the loss of inversion symmetry. Further
cooling results in the development of an incommensurate ferroelectric phase,
followed by a lock-in transition towards a twofold superstructure. Here we
present the incommensurately modulated crystal structure of Phz-H2ca at T =
139 K with a symmetry given by the superspace group P21(1/2 sigma2 1/2)0 and sigma2 =
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. 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, and much smaller variations of bond lengths of
atoms not involved in the resonance mechanism. 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.
