Tuning the magnetic properties of new layered iron chalcogenides (BaF)₂Fe_{2- x}Q₃ (Q = S, Se) by changing the defect concentration on the iron sublattice

Effecting and controlling ferromagnetic-like properties in semiconductors has proven to be a complex problem, especially when approaching room temperature. Here, we demonstrate the important role of defects in the magnetic properties of semiconductors by reporting the structures and properties of the iron chalcogenides (BaF)₂Fe_2-xQ₃ (Q = S, Se), which exhibit anomalous magnetic properties that are correlated with defects in the Fe-sublattice. The compounds form in both long-range ordered and disordered polytypes of a new structure typified by the alternate stacking of fluorite (BaF)₂²⁺ and (Fe_2-xQ₃)^2- layers. The latter layers exhibit an ordered array of strong Fe-Fe dimers in edge-sharing tetrahedra. Given the strong Fe-Fe interaction, it is expected that the Fe-Fe dimer is antiferromagnetically coupled, yet crystals exhibit a weak ferromagnetic moment that orders at relatively high temperature: below 280-315 K and 240-275 K for the sulfide and selenide analogues, respectively. This transition temperature positively correlates with the concentration of defects in the Fe-sublattice, as determined by single-crystal X-ray diffraction. Our results indicate that internal defects in Fe_2-xQ₃ layers play an important role in dictating the magnetic properties of newly discovered (BaF)₂Fe_2-xQ₃ (Q = S, Se), which can yield switchable ferromagnetically ordered moments at or above room temperature.