Multiferroicity due to charge ordering

In this contribution to the special issue on multiferroics we focus on multiferroicity driven by different forms of charge ordering. We will present the generic mechanisms by which charge ordering can induce ferroelectricity in magnetic systems. There is a number of specific classes of materials for which this is relevant. We will discuss in some detail (i) perovskite manganites of the type (PrCa)MnO₃, (ii) the complex and interesting situation in magnetite Fe₃O₄, (iii) strongly ferroelectric frustrated LuFe₂O₄ and (iv) an example of a quasi-one-dimensional organic system. All these are 'type-I' multiferroics, in which ferroelectricity and magnetism have different origins and occur at different temperatures. In the second part of this article we discuss 'type-II' multiferroics, in which ferroelectricity is completely due to magnetism, but with charge ordering playing an important role, such as (v) the newly discovered multiferroic Ca₃CoMnO₆, (vi) possible ferroelectricity in rare earth perovskite nickelates of the type RNiO₃, (vii) multiferroic properties of manganites of the type RMn₂O₅, (viii) perovskite manganites with magnetic E-type ordering and (ix) bilayer manganites.