  Beyond a conventional classification of ferroelectricity, there is a class of
materials where electronic degrees of freedom and electronic interactions are
directly responsible for electric polarization and ferroelectric transition.
This is termed electronic ferroelectricity. In this article, we review
electronic ferroelectricity from a view point of frustration. Experimental and
theoretical examinations in spin driven ferroelectric materials, recently
termed multiferroics, are introduced. Spin frustration caused by competing
magnetic interactions is of prime importance for this type of ferroelectricity.
Charge driven ferroelectricity where electronic charge order induces electric
polarization is reviewed. In particular, exotic dielectric and
magneto-dielectric properties in layered iron oxides are focused on. Through a
number of recent experimental and theoretical researches, charge fluctuation
due to frustrated geometry plays essential roles on electronic ferroelectricity
in this compound.
