ELECTRONIC AND SPIN STATE OF HIGH-VALENT IRON-OXIDES STUDIED BY A DV-X-ALPHA CLUSTER METHOD

The electronic and spin states of oxides containing highly charged Fe ions have been investigated by means of a DV-Xα cluster method. To study CaFe4+O3 and SrFe4+O3 in particular, model clusters FeO8-6, A8FeO8+6 (A = Ca and Sr), and Fe2O14-11 have been used, each being embedded in an appropriate Madelung potential. The direct contribution of the A cations to the valence state is almost insignificant. A covalent O2- → Fe4+ electron transfer considerably reduces the effective charges of these ions. A high-spin state of a typical d↑4.44d↓0.77 configuration is stabilized. The exchange interaction between a pair of Fe ions is predicted to change its sign from ferromagnetic to antiferromagnetic above a critical FeO distance of ∼1.93 Å. In a model cluster Fe2O14-11 made of two octahedra having different FeO bond lengths, a partial charge and spin transfer from the Fe ion in the contracted octahedron to the other in the expanded octahedron takes place, thus explaining the disproportionation transition of CaFeO3 formally expressed as 2Fe4+ ⇌ Fe3+ + Fe5+. © 1991.