First-principles investigation of boron defects in nickel ferrite spinel

The accumulation of boron within the porous nickel ferrite (NiFe2O4, NFO) deposited on fuel rods is a major technological problem with important safety and economical implications. In this work the electronic structure of nickel ferrite is investigated using first-principles methods, and the results are combined with experimental data to analyze B incorporation into the NFO structure. Under thermodynamic equilibrium the calculations predict that the incorporation of B into the NFO structure is unfavorable. The main limiting factors are the narrow stability domain of NFO and the precipitation of B2O3, Fe3BO5, and Ni3B2O6 as secondary phases. In n-type NFO, the most stable defect is Ni vacancy while in p-type material lowest the formation energy belongs to tetrahedrally coordinated interstitial B . Because of these limiting conditions it is more thermodynamically favorable for B to form secondary phases with Fe, Ni and O than it is to form point defects in NFO