  First-principles calculations based on density-functional theory in the
pseudo-potential approach have been performed for the total energy, crystal
structure and cell polarization for SrTaO$_2$N/SrTiO$_3$ heterostructures.
Different heterojunctions were analyzed in terms of the termination atoms at
the interface plane, and periodic or non-periodic stacking in the perpendicular
direction. The calculations show that the SrTaO$_2$N layer is compressed along
the $ab$-plane, while the SrTiO$_3$ is elongated, thus favoring the formation
of P4mm local environment on both sides of the interface, leading to net
macroscopic polarization. The analysis of the local polarization as a function
of the distance to the interface, for each individual unit cell was found to
depend on the presence of a N or an O atom at the interface, and also on the
asymmetric and not uniform $c$-axis deformation due to the induced strain in
the $ab$-plane. The resulting total polarization in the periodic array was $
\approx 0.54$ C/m$^2$, which makes this type of arrangement suitable for
microelectronic applications.
