Electronic structure and band gap of the negative charge-transfer material Sr3Fe2O7

We studied the electronic structure of the Sr3Fe2O7 compound using X-ray photoelectron spectroscopy (XPS). The charge-transfer satellites in the Fe 2p XPS spectra were analyzed using standard cluster model calculations. The analysis indicates that Sr3Fe2O7 is in the negative charge-transfer regime, and that the ground state is dominated by the 3d5L configuration (where L denotes an O 2p hole in the oxygen band). These results are similar to those found in the related SrFeO3 and Sr2FeO4 compounds. The band gap of the Sr3Fe2O7 compound is split off by the relatively large value of the p–d transfer integral Tσ. The lowest lying excitations are 3d5L+3d5L→3d5+3d5L2 and consequently the band gap is of the p–p type. The band gap in the Srn+1FenO3n+1 series can be understood taking into account the trend in the O 2p bandwidths.