Constants in Metals and Ionic Lattices

It has been indicated how the diffusion constant in a salt may be calculated from the ionic mobility of the current carrier (p. 268), or from the use of a radioactive isotope as an indicator (p. 244). Von Hevesy(3i) considers that it will be possible to use the latter method to follow the self-diffusion of numerous metals (Table 59). The method can be extended in a few instances by using as indicators small quantities of certain salts in solid solution in a closely related salt (77,78). For example, (78) small amounts of CuCl or NaCl were dissolved in AgCl. These mixtures are all cationic conductors in the temperature and concentration range investigated. The diffusion constants Z Na+ were measured, and also the conduc- 

Specific Ionic Mobility of Ag+ in pure AgCl Specific Ionic Mobility of Na+ in mixed crystal 

Then the above relation gives for the self-diffusion constant 9 X 10 and 21 x 10 cm. day respectively. The agreement is satisfactory. 

Those diffusion systems which have been marked with an asterisk in Table 67 represent metal pairs which form a continuous series of mixed crystals. As a rule in such systems the value of is small, an important exception being selfdiffusion processes (Pb in Pb iDq = 5-1 cm. sec. Au in Au Do = 1-26 X lO cm. sec. ). The normal range of values of Dq lies between 10 and 10 cm. sec. . There are exceptions, however for example, the diffusions of silicon and tin into copper give = 1-Ox 10 and 6-7 x 10 cm. sec. . Dq for the self-diffusion of bismuth, in a direction perpendicular to the c-axis, reaches a value of (1-33—16-3) x 10. It is difficult to assess the reproducibility of some of the data, since some of the diffusion processes listed are structure sensitive, but those for bismuth appear reasonably consistent (see Fig. 100). It is to be noted that the higher the 

Interdiffusiiig metals form a continuous series of solid solutions, t Bifiusion may show structure sensitivity (see next Chapter). 

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