For 64 atoms cell you can test nearest neighbor(NN) , 2nd NN, and so on. This can give you some local information about local arrangements of the defects.
If you reduce the cell to 32 and insert one dopant ion, then you are simulating an ordered array of defects because of the periodic boundary conditions. In fact the interaction across the cell boundary applies also to the case of 64 atoms above, but one hopes (and can possibly show) that the results represent the local interactions between the two defects within the cell rather than the image-image interaction across the cell boundary. In my opinion, a clever way to study concentrated solutions of defects is to resort to model Hamiltonians along the lines of the cluster expansion approach. This can work nicely if you care about neutral defects in semiconductors (eg: the oxygen vacancy in CeO2 as in http://arxiv.org/abs/1206.5429 ). For charged defects where long range interactions become important, there is no clean way (as far as I know) to get a model Hamiltonian for the interacting defects system. In the literature (especially that of CeO2) you may find few attempts for concentrated charged defects. See for example: http://journals.aps.org/prb/abstract/10.1103/PhysRevB.90.115120 Mostafa Youssef MIT
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