Ferroelectricity and Pressure-Induced Phenomena Driven by Neutral Ionic Valence Instability of Acid鈥揃ase Supramolecules

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• 作者：Reiji Kumai ; Sachio Horiuchi ; Jun Fujioka ; Yoshinori Tokura
• 刊名：The Journal of the American Chemical Society
• 出版年：2012
• 出版时间：January 18, 2012
• 年：2012
• 卷：134
• 期：2
• 页码：1036-1046
• 全文大小：879K
• 年卷期：v.134,no.2(January 18, 2012)
• ISSN：1520-5126

文摘

Supramolecular ferroelectric cocrystals of phenazine (Phz) with chloranilic acid (H₂ca), bromanilic acid (H₂ba), and fluoranilic acid (H₂fa) have been characterized by the interplay between their structural transformations and solid-state acid鈥揵ase (proton transfer) reactions. At ambient pressure, the Phz-H₂ca, Phz-H₂ba, 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 H₂fa, 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-H₂ca (FE-II form), Phz-H₂ba (FE-III form), and Phz-H₂fa (paraelectric form), their systematic changes under pressure depict a universal pressure鈥搕emperature 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.