  We used first-principles calculations to investigate the existence and origin
of the ferroelectric in- stability in the ABF3 fluoro-perovskites. We find that
many fluoro-perovskites have a ferroelectric instability in their high symmetry
cubic structure, which 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.
