  Large electron-electron Coulomb-interactions in correlated systems can lead
to a periodic arrangement of localized electrons, the so called "charge-order".
The latter is here proposed as a driving force behind ferroelectricity in iron
fluoride K0.6FeF3. By means of density functional theory, we propose different
non-centrosymmetric d5/d6 charge-ordering patterns, each giving rise to
polarization along different crystallographic axes and with different
magnitudes. Accordingly, we introduce the concept of "ferroelectric anistropy"
(peculiar to improper ferroelectrics with polarization induced by electronic
degrees of freedom), denoting the small energy difference between competing
charge-ordered states that might be stabilized upon electrical field-cooling.
Moreover, we suggest a novel type of charge-order-induced ferroelasticity:
first-principles simulations predict a monoclinic distortion to be driven by a
specific charge-ordering pattern, which, in turn, unambiguously determines the
direction of ferroelectric polarization. K0.6FeF3 therefore emerges as a
prototypical compound, in which the intimately coupled electronic and
structural degrees of freedom result in a manifest and peculiar
multiferroicity.
