A calculation of the interaction between impurity ions and grain boundaries in NiO

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Abstract
The substitutional energies of impurity ions close to coincidence grain boundaries in NiO, and in the bulk crystal, have been calculated. The interaction energies of a number of neutral impurities with the (211)/[011] tilt boundary were investigated, and were found to vary linearly, to a good approximation, with the ionic size misfit. Using these results the enhancements of the impurity concentrations at the boundary were estimated. The concentrations of aliovalent impurities and the compensating defects are also enhanced at coincidence grain boundaries. The differences between the concentrations of positively and negatively charged defects generally leads to a net charge density at the interface, which is compensated by a space charge layer. In the example considered here (Al2+ impurities compensated by singly charged vacancies) the boundary charge in the high-temperature intrinsic regime had the same sign as that in the low-temperature impurity-dominated region. Thus, in contrast to the analysis of Kliewer and Koehler (1965), there is no iso-electric temperature for coincidence boundaries in NiO with Al2+ impurities.