  Bad metal properties have motivated a description of the parent iron
pnictides as correlated metals on the verge of Mott localization. What has been
unclear is whether interactions can push these and related compounds to the
Mott insulating side of the phase diagram. Here we consider the iron
oxychalcogenides La2O2Fe2O(Se,S)2, which contain an Fe square lattice with an
expanded unit cell. We show theoretically that they contain enhanced
correlation effects through band narrowing compared to LaOFeAs, and we provide
experimental evidence that they are Mott insulators with moderate charge gaps.
We also discuss the magnetic properties in terms of a Heisenberg model with
frustrating J1-J2-J2' exchange interactions on a "doubled" checkerboard
lattice.
