High-Temperature and Pressure-Induced Ferroelectricity in Hydrogen-Bonded Supramolecular Crystals of Anilic Acids and 2,3-Di(2-pyridinyl)pyrazine
2013
Horiuchi, Sachio | Kumai, Reiji | Tokura, Yoshinori
Cocrystallization of anilic acids (H₂xa) and 2,3-di(2-pyridinyl)pyrazine (dppz) affords a variety of molecular geometries, including hydrogen-bonding and supramolecular structures. Proton-transferred 1:1 salts of [H-dppz][Hca] and [H-dppz][Hba] (H₂ca = chloranilic acid, H₂ba = bromanilic acid) were found to host room-temperature ferroelectricity with a spontaneous polarization of 3–4 μC/cm² along the hydrogen-bonded chains. Compared with the Curie points of other supramolecular ferroelectrics, those of the salts are relatively high (402 K and >420 K, respectively) because of the elongated hydrogen bonds, which stabilize the proton-ordered state against thermal agitation. In addition to the ferroelectric black (α) form, dppz and H₂ba gave two different crystal forms with a 2:3 ratio: the brown β form of [H₁.₅-dppz]₂[Hba]₃ and the brownish-red γ form of [H-dppz]₂[Hba]₂[H₂ba]. Mixed solutions of dppz with the less acidic fluoranilic acid (H₂fa) exhibit valence instability; the H₂fa molecules remain mostly neutral in absolute ethanol, whereas methanol (MeOH) solution apparently increases the deprotonated Hfa– content. Crystallizations of these solutions gave a neutral [dppz][H₂fa] cocrystal and ionic [H-dppz⁺][Hfa–]·MeOH salt, respectively. The ferroelectricity induced by a modest hydrostatic pressure corroborates the conclusion that the ionic state with a dipolar [H-dppz⁺][Hfa–] chain is energetically close to the nonpolar neutral ground state of the [dppz][H₂fa] crystal.
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