  The ability to tune the iron chalcogenides BaFe2Q3 from Mott insulators, to
metals and then superconductors with applied pressure has renewed interest in
low-dimensional iron chalcogenides and oxychalcogenides. We report here a
combined experimental and theoretical study on the iron oxychalcogenides
BaFe2Q2O (Q=S, Se) and show that their magnetic behaviour results from
nearest-neighbour magnetic exchange interactions via oxide and selenide anions
of similar strength, with properties consistent with more localised electronic
structures than those of BaFe2Q3 systems.
