Latent electronic (anti-)ferroelectricity in BiNiO$_3$

BiNiO$_3$ exhibits an unusual metal-insulator transition from $Pnma$ to $P\overline{1}$ that is related to charge ordering at the Bi sites, which is intriguingly distinct from the charge ordering at Ni sites usually observed in related rare-earth nickelates. Here, using first principles calculations, we first rationalize the phase transition from $Pnma$ to $P\overline{1}$, revealing an overlooked intermediate $P2_1/m$ phase and a very unusual phase transition mechanism. Going further, we point out that the charge ordering at Bi sites in the $P\overline{1}$ phase is not unique. We highlight an alternative polar orderings giving rise to a ferroelectric $Pmn2_1$ phase nearly degenerated in energy with $P\overline{1}$ and showing an in-plane electric polarisation of 53 $\mu C$/cm$^2$ directly resulting from the charge ordering. The close energy of $Pmn2_1$ and $P\overline{1}$ phases, together with low energy barrier between them, make BiNiO$_3$ a potential electronic antiferroelectric in which the field-induced transition from non-polar to polar would relate to non-adiabatic inter-site electron transfer. We also demonstrate the possibility to stabilize an electronic ferroelectric ground state from strain engineering in thin films, using an appropriate substrate