  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
