Structure and physical properties of the layered iron oxychalcogenide BaFe₂Se₂O

We have successfully synthesized a layered iron oxychalcogenide BaFe ₂Se₂O single crystal. This compound is built up of Ba and Fe-Se(O) layers alternatively stacked along the c axis. The Fe-Se(O) layers contain double chains of edge-shared Fe-Se(O) tetrahedra that propagate along the b axis and are bridged by oxygen along the a axis. Physical property measurements indicate that BaFe₂Se₂O is a semiconductor without the Curie-Weiss behavior up to 350 K. There is a possible long-range antiferromagnetic transition at 240 K, corresponding to the peak in specific-heat measurement, and two transitions at 115 K and 43 K where magnetic susceptibility drops abruptly. The magnetic entropy up to 300 K is much smaller than the expected value for Fe2⁺ in tetrahedral crystal fields and the Mössbauer spectrum indicates that long-range magnetic order is unlikely at 294 K. Our results suggest that BaFe₂Se₂O is a magnetic insulator at the borderline between a long-range antiferromagnetic spin ordering and possible spin dimerization.