APS/Matthew Dawber/Alan Stonebraker

Figure 1: Schematics illustrating the contrasting role of electronic ferroelectricity in $BaTiO3$ and TTF-CA. (a) The cubic unit cell of $BaTiO3$ is shown with $Ti$ in green, $Ba$ in blue, and $O$ in red. Some of the $2p$ electron orbitals for the oxygen ions are drawn schematically in light blue. (b) Below $120∘C$, the $Ti$ ion shifts upwards relative to the other ions, creating a dipole, $p⃗ion$. The electronic weight in the oxygen $2p$ orbitals also shifts but, in this case, downwards, resulting in an additional electronic polarization, $p⃗ele$. (c) The TTF-CA crystal structure is composed of alternating stacks of TTF and CA molecules. In between, the hybridized electron orbitals are represented in light blue. (d) Below $81K$, the TTF molecules donate an electron to nearest neighboring CA molecule, and at the same time, shift upwards, producing an ionic polarization. A corresponding upward shift in the electronic weight of the hybridized orbitals creates a strong electronic dipole that points in the opposite direction to the ionic one.