Theoretical Characterizations of Spinels Containing Iron and Vanadium via ab initio Calculations

抄録

We have employed ab initio approaches to investigate normal and inverse spinels containing iron and vanadium. The valence states of tetrahedral and octahedral Fe and V were firstly calibrated with reference oxides FeO (rocksalt), Fe₂O₃ (corundum), VO (rocksalt) and Fe₂O₃ (corundum). The Mulliken charges analyses suggest the valence states of Fe and V are interstices dependent. In Fe rich condition (VFe₂O₄), bivalent cations Fe²⁺ and V²⁺ prefer tetrahedral interstices, while trivalent cations Fe³⁺ and V³⁺ prefer octahedral interstices. In V rich condition (FeV₂O₄), Fe valence states in tetrahedral and octahedral interstices are the same as those in Fe rich cases. However, the V cations have contrary valence states, namely, V³⁺, in tetrahedral interstices, and mixed valence states in octahedral states.
The crystalline formation energies of normal and inverse spinels were addressed to determine their stability. The inverse spinels are obviously more favorable than normal spinels. We have quantified probability of two isomeric inverse spinels in Fe rich condition in equilibrium at 300 K and 1700 K. It is in agreement with that entropy plays a more significant role at high temperature. Electronic structures of tetrahedral and octahedral Fe and V cations have also been analyzed using computed x-ray absorption near edge structure (XANES). The chemical shift of white lines, going from Fe²⁺ to Fe³⁺ cations in spinels, and 3d orbitals splitting of tetrahedral and octahedral V cations are distinguishable in XANES spectra. Thus, the different electronic structure of Fe and V cations in tetrahedral and octahedral interstices can provide important interpretations of experimental works of spinels containing iron and vanadium elements.