Role of Crystal Structure in Diffusion. I. Diffusion Paths in Closest-Packed Crystals

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Abstract
A consideration of the crystal structures of simple binary compounds shows that they can be represented by closest packings of the larger anions in which two kinds of interstices are available for occupation by the metal atoms. In hexagonal closest packings, the octahedral voids form continuous chains by sharing opposite faces while the tetrahedral voids form isolated pairs. In cubic closest packings, each kind of void shares faces only with unlike voids. The specific diffusion paths available in these compounds depend, therefore, on the manner in which the voids are occupied. Continuous diffusion paths comprised of normally unoccupied voids exist in ZnO and α-ZnS type structures so that voidal diffusion can take place without requiring defect formation. Similarly, voidal diffusion can occur in BiO3, CrCl3, and CdI2 type structures. Conversely, all possible continuous diffusion paths are blocked by metal atoms in the NiAs, NaCl, and antifluorite-type structures so that vacancy or interstitialcy mechanisms are necessary to account for diffusion.