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The promotion of n ‐type electronic conduction by relatively small concentrations of donors in the presence of a large concentration of calcium, which is an acceptor, is explained by assuming that most of the calcium is present as neutral calcium ion‐oxygen vacancy pairs or clusters. Evaluation of experimental data on the basis of this model leads to an enthalpy of pairing of calcium ions with oxygen ion vacancies of –2.7 ev.
F. A. Kröger (1966) studied this question.