130 ADVEXXrilES IN E.\DIOISOTOPE RESEARCH 



THE DIFFUSION OF LEAD IONS IN LEAD IODIDE 



Lead iodide has a smaller electrolytic conductance llian lead chloride ; 

 yet a relatively high diffusion velocity of lead ions in lead iodide would 

 be expected, in spite of the low conductance, since Tubandt found a 

 high value (0.67) for the transport num])er of the lead ion in Pblg. In 

 agreement with this expectation, it is evident from Table 2 that the 

 diffusion constant can be measured even a few degrees above 100°(J. 

 It was always necessary to use annealed pellets in order to obtain repro- 

 ducible A^alues. 



We find the molecular heat of relaxation to have a value of 30,000 

 cal and the constant A to amount to 3.43 10^. 



The diil'usion constant can be represented by the formula 



/) z= 3.43 X 10^ e 30.000/flr 



Since there is an appreciable mobility of the lead ion in Pbig even a1 

 temperatures which are very far removed from the melting point it was 

 also possil)le to apply the decrease of ionization after diffusion, due to 

 the a-particles, for measuring the diffusion constant. The results are 

 shown in Table 3 and Fig. 2. The diffusion constant can be represented 

 ))y table following equation : 



Z) = 9.11 X 10^ e-3oi40/7?r 



The heal of relaxation, which amounts to 30,140 cal/mole, does not 

 difler appreciably from the value yielded l)y the recoil experiments 

 (30,000 cal). This agreement is also expressed in the parallel courses of 

 the curves in Fig. 2. The fact that they do not quite coincide is probably 

 due to some uncertainty attaching to the value of the recoil range, as 

 already mentioned above. The two straight lines could be superposed by 

 assuming the range of the recoil particles in lead iodide to be 0.11 // 

 instead of 0.075 //. 



