The structural, electronic, and magnetic properties of the O-vacancy-ordered mixed-valence Sr4Fe4O11 phase were investigated by using spin-polarized electronic-structure total-energy calculations. The optimized structural parameters obtained from accurate total-energy calculations were found to be in very good agreement with low-temperature neutron-diffraction findings. Among the different spin configurations considered for Sr4Fe4O11, the G-type antiferromagnetic configuration was found to represent the magnetic ground state. The calculations revealed finite magnetic moments at both Fe sites. This was contrary to the conclusions drawn from Mössbauer spectroscopy and low-temperature powder neutron-diffraction measurements, but was consistent with the results of magnetization measurements. The present study clearly showed that one of the magnetic sub-lattices was frustrated and, hence, that Sr4Fe4O11 could be considered to be a phase-separated system with one phase in the G-type antiferromagnetic state and the other in a spin-glass-like state. The theoretical results showed unambiguously that Fe atoms with a square pyramidal environment had a lower oxidation state than one in octahedral coordination. However, the presence of a covalent interaction between Fe and the neighboring O atoms made the actual oxidation state considerably lower than the formal oxidation state of 3+ and 4+ for square-pyramidal and octahedral coordinations, respectively. The Sr4Fe4O11 was found to be semiconducting in the antiferromagnetic ground state.

Spin- and Charge-Ordering in Oxygen-Vacancy-Ordered Mixed-Valence Sr4Fe4O11. R.Vidya, P.Ravindran, H.Fjellvåg, A.Kjekshus: Physical Review B, 2006, 74[5], 054422 (10pp)