  New Fe-pnictide heterostructures of the type LnOFeAs/BaFe$_2$As$_2$ (Ln = La,
Sm) were grown by pulsed laser deposition (PLD) and investigated. Their common
structural unit of [Fe$_2$As$_2$] planes allows perfect matching between the
different Fe-pnictide unit cells and a coherent and atomically sharp interface.
We test the stability of the heterointerface in the presence of Co$^{2+}$
(cations) as well as for excess O$^{2-}$ (anions) and discuss the consequences
on the electronic properties: While undoped SmOFeAs/BaFe$_2$As$_2$ remains
non-superconducting, a balanced Co-concentration after diffusion across the
interface results in superconductivity within Co-substituted variants. In
contrast, excess O$^{2-}$ causes the formation of an interfacial layer in
SmOFeAs/BaFe$_2$As$_2$ with increased O$^{2-}$/As$^{3-}$ ratio and develops a
metal-to-superconductor transition with time. The engineered heterointerfaces
may provide a sophisticated pathway to bridge the gap between Fe-pnictides and
Fe-chalcogenides.
