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24), and (26) gives, for Dv, eB K ee D v = 1 + --xD' e1 Therefore, in a chemical diffusion problem in ionic solids, one must carefully distinguish between the intrinsic vacancy diffusion coefficient, B i , a constant, and the phenomenological vacancy diffusion coefficient, Dy. By assuming some presumably realistic values for the parameters in Eqs. (23) and (27), the variation of the vacancy diffusion coefficient and the sintering flux with impurity concentration can be illustrated. Equation (23) may be written to put it into the usual form.'O The results are given in Table I. Admittedly, the small values of X D are not consistent with some of the earlier assumptions but this does not detract from the overall qualitative changes in D v and Jt.

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