Bandgap narrowing in the layered oxysulfide semiconductor Ba_3Fe_2O_5Cu_2S_2: Role of FeO_2 layer

A new layered Cu-based oxychalcogenide Ba₃Fe₂O₅Cu₂S₂ has been synthesized and its magnetic and electronic properties were revealed. Ba₃Fe₂O₅Cu₂S₂ is built up by alternatively stacking Cu₂S₂]^2- layers and iron perovskite oxide (FeO₂)(BaO)(FeO₂)]^2- layers along the c axis that are separated by barium ions with Fe³⁺ fivefold coordinated by a square-pyramidal arrangement of oxygen. From the bond valence arguments, we inferred that in layered CuCh-based (Ch = S, Se, Te) compounds the +3 cation in perovskite oxide sheet prefers a square pyramidal site, while the lower valence cation prefers the square planar sites. The studies on susceptibility, transport, and optical reflectivity indicate that Ba₃Fe₂O₅Cu₂S₂ is an antiferromagnetic semiconductor with a Néel temperature of 121 K and an optical bandgap of 1.03 eV. The measurement of heat capacity from 10 K to room temperature shows no anomaly at 121 K. The Debye temperature is determined to be 113 K. Theoretical calculations indicate that the conduction band minimum is predominantly contributed by O 2p and 3d states of Fe ions that antiferromagnetically arranged in FeO₂ layers. The Fe 3d states are located at lower energy and result in a narrow bandgap in comparison with that of the isostructural Sr₃Sc₂O₅Cu₂S₂.