Electronic structure of the negative charge-transfer material Sr3FeMO7 (M=Fe, Co, Ni)

We studied the electronic structure of the substituted Sr3FeMO7 compounds (M=Fe, Co, Ni). The experimental technique used in the study was M 2p X-ray photoelectron spectroscopy (XPS). The charge-transfer satellites in the M 2p spectra were analyzed using cluster model calculations. The analysis indicates that these materials are in the negative charge-transfer regime, and that their ground states are dominated by the 3dn+1L configuration (where L denotes a 2p hole in the oxygen band). The results are similar to those found in the closely related SrFeO3 and SrCoO3 compounds. The band gap of these compounds is split off by the relatively large value of the p-d transfer integral Tσ The lowest lying excitations are 3dn+1L+3dn+1L -> 3dn+1+3dn+1L2 and the band gap is of the p-p type. The value of the p-d transfer integral Tσ in the Sr3FeMO7 compounds decreases from Fe to Ni. In turn, this helps to explain the increase in the electrical conductivity along the M=Fe, Co, Ni series