  We investigate the effect of polar Sr-O vacancy pairs on the electric
polarization of SrMnO$_3$ (SMO) thin films using density functional theory
(DFT) calculations. This is motivated by indications that ferroelectricity in
complex oxides can be engineered by epitaxial strain but also \textit{via} the
defect chemistry. Our results suggest that intrinsic doping by cation and anion
divacancies can induce a local polarization in unstrained non-polar SMO thin
films and that a ferroelectric state can be stabilized below the critical
strain of the stoichiometric material. This polarity is promoted by the
electric dipole associated with the defect pair and its coupling to the atomic
relaxations upon defect formation that polarize a region around the defect.
This suggests that polar defect pairs affect the strain-dependent
ferroelectricity in semiconducting antiferromagnetic SMO. For metallic
ferromagnetic SMO we find a much weaker coupling between the defect dipole and
the polarization due to much stronger electronic screening. Coupling of
defect-pair dipoles at high enough concentrations along with their switchable
orientation thus makes them a promising route to affect the ferroelectric
transition in complex transition metal oxide thin films.
