  Recent experiments have reported that inserting metal atoms or small
molecules in between the FeSe layers of $\beta$-FeSe can significantly enhance
the superconducting transition temperature. Here, based on first-principles
electronic structure calculations, we propose a stable compound SnOFeSe by
alternatively stacking the SnO and $\beta$-FeSe layers. The predicted SnOFeSe
has the same tetragonal structure as the well-known FeAs-based compound
LaOFeAs, meanwhile their electronic structures in the nonmagnetic state are
quite similar. The magnetic ground state of SnOFeSe is predicted to be the
dimer antiferromagnetic (AFM) state, which is energetically only 2.77 (2.15)
meV/Fe lower than the trimer (dimer-trimer-dimer-trimer) AFM state, indicating
that strong magnetic fluctuations might be induced via slight modulation.
Interestingly, SnOFeSe is at the verge of metal-insulator transition in these
low-energy magnetic states, hence bridging the metallic parent compounds of
iron-based superconductors and the insulating ones of cuprate superconductors.
With the reduced dimensionality, monolayer SnOFeSe also shows great
similarities in the electronic and magnetic properties to its bulk phase. Given
that SnOFeSe is adjacent to magnetic frustration and resembles LaOFeAs in both
crystal and electronic structures, we suggest that SnOFeSe is a possible
superconductor parent compound, which may provide a promising platform to study
the interplay between magnetism and unconventional superconductivity in
FeSe-derived materials.
