Geometric ferroelectricity in fluoroperovskites

We used first-principles calculations to investigate the existence and origin of the ferroelectric instability in
the ABF3 fluoroperovskites. While the ground states of most ABF3 compounds are paraelectric (Pnma phase),
we find that many fluoroperovskites have a ferroelectric instability in their high-symmetry cubic structure that
is of similar amplitude to that commonly found in oxide perovskites. In contrast to the oxides, however, the
fluorides have nominal Born effective charges, indicating a different mechanism for the instability.We show that
the instability originates from ionic size effects, and is therefore in most cases largely insensitive to pressure
and strain, again in contrast to the oxide perovskites. An exception is NaMnF3, where coherent epitaxial strain
matching to a substrate with equal in-plane lattice constants destabilizes the bulk Pnma structure, leading to a
ferroelectric, and indeed multiferroic, ground state with an unusual polarization/strain response.