Consiglio Nazionale delle Ricerche (CNR-SPIN), 67100 L'Aquila, Italy.
Phys Rev Lett. 2010 Sep 3;105(10):107202. doi: 10.1103/PhysRevLett.105.107202. Epub 2010 Sep 1.
Charge order is proposed as a driving force behind ferroelectricity in iron fluoride K(0.6)Fe(0.6)(II)Fe(0.4)(III)F(3). By means of density functional theory, we propose several noncentrosymmetric d(5)/d(6) charge-ordering patterns, each giving rise to polarization with different direction and magnitude. Accordingly, we introduce the concept of "ferroelectric anisotropy" (peculiar to improper ferroelectrics with polarization induced by electronic degrees of freedom), denoting the small energy difference between competing charge-ordered states. Moreover, we suggest a novel type of charge-order-induced ferroelasticity: a monoclinic distortion is induced by a specific charge-ordering pattern, which, in turn, determines the direction of polarization. K(0.6)Fe(0.6)(II)Fe(0.4)(III)F(3) therefore emerges as a prototypical compound, in which the intimately coupled electronic and structural degrees of freedom result in a peculiar multiferroicity.
电荷有序被认为是氟化铁 K(0.6)Fe(0.6)(II)Fe(0.4)(III)F(3)中铁电现象的驱动力。通过密度泛函理论，我们提出了几种非中心对称的 d(5)/d(6)电荷有序模式，每种模式都会产生具有不同方向和大小的极化。因此，我们引入了“铁电各向异性”的概念（这是由电子自由度引起的非中心铁电体所特有的），表示竞争电荷有序态之间的小能量差。此外，我们还提出了一种新的电荷有序诱导铁弹性：由特定的电荷有序模式诱导单斜畸变，进而决定极化的方向。因此，K(0.6)Fe(0.6)(II)Fe(0.4)(III)F(3)成为一种典型的化合物，其中电子和结构自由度的紧密耦合导致了一种特殊的多铁性。
