  By performing first-principles calculations on four capacitor structures
based on BaTiO3 and PbTiO3, we determine the intrinsic interfacial effects that
are responsible for the destabilization of the polar state in thin-film
ferroelectric devices. We show that, contrary to the established
interpretation, the interfacial capacitance depends crucially on the local
chemical environment of the interface through the force constants of the
metal-oxide bonds, and is not necessarily related to the bulk screening
properties of the electrode material. In particular, in the case of interfaces
between AO-terminated perovskites and simple metals, we demonstrate a novel
mechanism of "interfacial ferroelectricity" that produces an overall
enhancement of the ferroelectric instability of the film, rather than a
suppression as is usually assumed. The resulting "negative dead layer" suggests
a route to thin-film ferroelectric devices that are free of deleterious size
effects.
