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Mechanism of ferroelectric instabilities in non d^0 perovskites: LaCrO_3 versus CaMnO_3

The incompatibility of partial d occupation on the perovskite B-site with the standard charge transfer mechanism for ferroelectricity has been a central paradigm in multiferroics research. Nevertheless, it was recently shown by density functional theory calculations that CaMnO_3 exhibits a polar instability that even dominates over the octahedral tilting for slightly enlarged unit cell volume. Here, we present similar calculations for LaCrO_3, which has the same d^3 B-site electron configuration as CaMnO_3. We find that LaCrO_3 exhibits a very similar, albeit much weaker, polar instability as CaMnO_3. In addition, while the Born effective charge (BEC) of the Mn^{4+} cation in CaMnO_3 is highly anomalous, the BEC of Cr^{3+} in LaCrO_3 is only slightly enhanced. By decomposing the BECs into contributions of individual Wannier functions we show that the ferroelectric instabilities in both systems can be understood in terms of charge transfer between TM d and O p states, analogously to the standard d^0 perovskite ferroelectrics.