Ferroelectricity and pressure-induced phenomena driven by neutral ionic valence instability of acid-base supramolecules.
Ferroelectricity and pressure-induced phenomena driven by neutral ionic valence instability of acid-base supramolecules.
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DOI：
10.1021/ja208113p
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发表时间：
2012-01
影响因子：
15
通讯作者：
R. Kumai;S. Horiuchi;J. Fujioka;Y. Tokura
中科院分区：
文献类型：
--
作者：
R. Kumai;S. Horiuchi;J. Fujioka;Y. Tokura
吩嗪（Phz）与氯冉酸（H（2）ca）、溴邻氨基苯甲酸（H（2）ba）和氟对氨基苯甲酸（H（2）fa）的超分子铁电共晶的结构转变和固态酸碱（质子转移）反应之间的相互作用已被表征。在常压下，Phz-H（2）ca、Phz-H（2）ba及其氘代晶体表现出不完全的质子置换，这在低于居里点的低温下将中性分子转变为半离子（T（c）（IC）< T < T（c）（I））。对于酸性较低的H（2）fa的共晶，铁电相仅通过施加高于~0.6 GPa的流体静压来诱导。根据介电常数随温度的变化和同步辐射X射线衍射的结合研究，证明了铁电（FE-I）相在较低温度下总是伴随着向具有非公度周期的晶格调制相的连续相变（IC相，T（c）（II）< T < T（c）（IC））和具有相称的（2倍或3倍）周期性（FE-II或FE-III相，T < T（c）（II））。虽然在常压下的基态结构在Phz-H（2）ca（FE-II形式）、Phz-H（2）ba（FE-III形式）和Phz-H（2）fa（顺电形式）中彼此不同，但它们在压力下的系统变化描绘了一个普适的压力-温度相图。结构变化的可能起源被分配到价不稳定性和挫折库仑相互作用，诱导电荷pectionation（共存的中性离子）状态与分期空间秩序。
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.