APS/Matthew Dawber/Alan Stonebraker
Schematics illustrating the contrasting role of electronic ferroelectricity in and TTF-CA. (a) The cubic unit cell of is shown with in green, in blue, and in red. Some of the electron orbitals for the oxygen ions are drawn schematically in light blue. (b) Below , the ion shifts upwards relative to the other ions, creating a dipole, . The electronic weight in the oxygen orbitals also shifts but, in this case, downwards, resulting in an additional electronic polarization, . (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 , 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.