Figure 1
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, pion. The electronic weight in the oxygen 2p orbitals also shifts but, in this case, downwards, resulting in an additional electronic polarization, pele. (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 81 K, 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.