Ferroelectricity and Pressure-Induced Phenomena Driven by Neutral Ionic Valence Instability of Acid–Base Supramolecules

Abstract

Supramolecular ferroelectric cocrystals of phenazine (Phz) with chloranilic acid (H(2)ca), bromanilic acid (H(2)ba), and fluoranilic acid (H(2)fa) have been characterized by the interplay between their structural transformations and solid-state acid-base (proton transfer) reactions. At ambient pressure, the Phz-H(2)ca, Phz-H(2)ba, and their deuterated crystals exhibit incomplete proton displacement, which transforms the neutral molecules into semi-ionic at low temperatures below the Curie point (T(c)(IC) < T < T(c)(I)). For the cocrystal of the less acidic H(2)fa, the ferroelectric phase is induced only by applying hydrostatic pressure above ~0.6 GPa. According to the combined studies of temperature-dependent dielectric permittivity and synchrotron X-ray diffraction, it was proved that the ferroelectric (FE-I) phase is always accompanied at lower temperatures by successive phase transitions to the lattice modulated phases with incommensurate periodicities (IC phase, T(c)(II) < T < T(c)(IC)) and with commensurate (2- or 3-fold) periodicities (FE-II or FE-III phase, T < T(c)(II)). Whereas the ground-state structures at ambient pressure are different from one another among the Phz-H(2)ca (FE-II form), Phz-H(2)ba (FE-III form), and Phz-H(2)fa (paraelectric form), their systematic changes under pressure depict a universal pressure-temperature phase diagram. The possible origins of structural changes are assigned to the valence instability and the frustrated Coulomb interactions that induce the charge disproportionation (coexisting neutral ionic) states with the staging spatial orders.

Resonance-stabilized partial proton transfer in hydrogen bonds of incommensurate phenazine–chloranilic acid

“…Strong support for proton transfer in part of the hydrogen bonds has been obtained through the variations of the lengths of precisely those bonds that are involved in resonance stabilization of the Hca À ion (x3.2). Proton transfer in only part of the hydrogen bonds has been explained as the result of Coulomb interactions between the resulting ionic species (Kumai et al, 2012). Proton transfer is in line with the acidities of the two molecules with pK a1 = 1.23 for the proton acceptor Phz, and pK a1 = 0.76 for the proton donor H 2 ca (Albert & Phillips, 1956;Molcanov & Prodic, 2010).…”

“…Ferroelectricity in Phz-H 2 ca at low temperatures is the result of intermolecular proton transfer within the O1-H1o1Á Á ÁN1 hydrogen bonds (Horiuchi, Kumai & Tokura, 2005;Kumai et al, 2007Kumai et al, , 2012Gotoh et al, 2007;Noohinejad et al, 2014). Consideration of the positions of the H atoms within the O1-H1o1Á Á ÁN1 hydrogen bonds of the crystal structures of the three phases leads to the following model for the phase transitions.…”

“…Organic materials based on hydrogen-bonded supramolecular chains with a polar space group form one class of ferroelectric materials. The co-crystal of phenazine (Phz) and 2,5-dichloro-3,6-dihydroxy-p-benzoquinone (chloranilic acid, H 2 ca) is one of several recently discovered hydrogen-bonded organic ferroelectrics (Horiuchi, Ishii et al, 2005;Horiuchi et al, 2009;Kumai et al, 2006Kumai et al, , 2012.…”

Multicomponent organic crystals at high pressure

“…Many of these systems are of practical importance, for example, for pharmaceutical industry [17][18][19][20][21][22][23][24][25], as ferroelectrics [26,27], non-linear optical materials [28,29], or as energetic materials [30,31]. At the same time, they are of great interest for basic understanding of the factors that influence the formation of crystal structures from fluid phases under different experimental conditions and their response to variations of temperature and pressure.…”

“…However, for a co-crystal of glutaric acid with glycine such a transfer, though could be expected, was not observed -since a conformational polymorphic transition takes place at a much lower pressure than would be required for such a transfer [50]. As other examples of the systems for which the interplay between the structural transformations and solid-state acid-base (proton transfer) reactions at high pressure has been studied in relation to the paraelectric and ferroelectric properties, one can mention the co-crystals of phenazine with chloranilic, bromanilic, and fluoranilic acids [26], or of co-crystals of anilic acids and 2,3-Di(2-pyridinyl)pyrazine [27], where hydrogen bonds are polarized and neutral molecules are converted into ionized species on cooling down, or on increasing pressure.…”