Originally published in Z. f. Elektrochem. 37, 52S (1931 



13. DIFFUSION IN METALS 



G. Hevesy and W. Seith 

 From the Institute of Physical Chemistry, Uni\ersity of Freiburg 



Diffusion in salt-like compounds is facilitated by means of a relaxation 

 process which occurs when the crystal is heated. This relaxation which 

 exhibits some similarity to activation in the theory of reaction velocity 

 depends chiefly on the size, valency, electron affinity and polarization 

 properties of the lattice components. In silver iodide, for example, 

 where the small univalent strongly-polarizing silver ion contrasts with the 

 large iodide ion which has slight attraction for electrons and is easily 

 polarizable, there is easy detachment of the silver ion and it is well- 

 known that mobilities indeed exceed those occurring in aqueous solutions. 

 In the pure metal the behaviour is altogether different. In such case 

 there is only one kind of lattice component, a high co-ordination number 

 and a high symmetry of charge distribution. Large diffusion velocities 

 cannot therefore be expected in pure metals. Metal alloys are different. 

 In the lead-gold system, for example, the small gold atom which has a 

 high affinity for the valence electron contrasts with the larger lead atom 

 which has a lower electron affinity, and hence there occurs a system which 

 is readily subject to relaxation in which the gold atom easily vacates 

 its position. Roberts- Austen in his classical investigations has 

 already been able to demonstrate that gold diffuses into lead even at 

 moderate temperatures with a considerable velocity. The velocity of 

 diffusion of gold in lead is attained through the speed of dissolution of 

 gold atoms in the gold-lead phase and in its taking up a new position. 

 The gold penetrates into lead but, on the contrary, lead is practically 

 immobile in gold. The diffusion constant for gold in lead is 4x10^3 

 cm^ day~i at 150° whereas the diffusion of lead in gold-lead at a temper- 

 ature of 141° amounts only to SxlO^^^ cm^ day^^, i.e. it is smaller by 

 seven orders of magnitude. Lead atoms accommodated near silver atoms 

 are more easily dissolved than those considered above. It has been 

 found that lead diffuses about twice as quickly in silver saturated with 

 lead as in pure lead. 



If the gold in lead alloys is replaced by other elements whose properties 

 become more and more similar to those of lead, then these elements show a 



