Structural, electronic properties and inter-atomic bonding in layered iron–pnictide oxides (Fe2As2)(Sr4O6), where M are Mg and Ti

Using the ab initio FLAPW-GGA method we have examined (Fe2As2)(Sr4 M 2 O6) phases (where M are Ti and Mg), to analyze the effect of replacements of d metals by sp elements in perovskite-type blocks as an effective way for the tuning of the electronic and structural properties of the recently synthesized 21113-type phases. We have found that unlike (Fe2As2)(Sr4Ti2O6), and (Fe2As2)(Sr4Mg2O6), where both building blocks are conducting, for (Fe2As2)(Sr4MgTiO6), conduction will happen exclusively in the [Fe2As2] blocks, whereas insulating [Sr4MgTiO6] blocks behave as “charge reservoirs” – as in other Fe-based superconductors. These data together with a set of so-called structural indicators (the a axis length, As Fe As bond angles, and anion height Δz) allow one to conclude that (Fe2As2)(Sr4MgTiO6) will be a promising superconducting material – in agreement with available experiments.