  The surface charge associated with the spontaneous polarization in
ferroelectrics is well known to cause a depolarizing field that can be
particularly detrimental in the thin-film geometry desirable for
microelectronic devices. Incomplete screening of the surface charge, for
example by metallic electrodes or surface adsorbates, can lead to the formation
of domains, suppression or reorientation of the polarization, or even
stabilization of a higher energy non-polar phase . A huge amount of research
and development effort has been invested in understanding the depolarizing
behavior and minimizing its unfavorable effects. Here we demonstrate the
opposite behavior: A strong polarizing field that drives thin films of
materials that are centrosymmetric and paraelectric in their bulk form into a
non-centrosymmetric, polar state. We illustrate the behavior using density
functional computations for perovskite-structure potassium tantalate, KTaO$_3$,
which is of considerable interest for its high dielectric constant, proximity
to a quantum critical point and superconductivity. We then provide a simple
recipe to identify whether a particular material and film orientation will
exhibit the effect, and develop an electrostatic model to estimate the critical
thickness of the induced polarization in terms of well-known material
parameters. Our results provide practical guidelines for exploiting the
electrostatic properties of thin-film ionic insulators to engineer novel
functionalities for nanoscale devices.
