Originally published in Z. Phijs. 79. 197 (1932) 



12. THE HEAT OF RELAXATION OF THE LEAD 



LATTICE 



G. Hevesy, W. Seith and A. Keil 

 From the Institute of Physical Chemistry, University- of Freiburg 



The heat of relaxation of the lead lattice (the work of release of the lead 

 atoms) is determined from the temperature coefficient of the velocity of 

 diffusion of a radioactive lead isotope in lead ; a study is made of the 

 sensitivity to structure of this quantity and the velocity of diffusion. 

 Besides the heat of vaporization, heat of fusion and lattice energy 

 there is another quantity of energy which is important to the crystalline 

 state of aggregation. This is the heat of relaxation of the crystal lattice, 

 or the energy of release of the lattice components. The latter is the energy 

 of activation of self- reaction, which takes place between the atoms of 

 a metal and results in place exchange of the lattice units. ^Vhen the 

 transport number is known the heat of relaxation of ions of an electro- 

 lytic conductor can be calculated from the temperature coefficient of 

 the conductivity. Direct measurement of the velocity of self-diffusion 

 is the only course open in dealing with metals. The heat of relaxation, 

 Q, is calculated from the diffusion constant, Z), measured at various 

 temperatures, by using the well-known formula 



D = .4e-<? '^^ 



where J. is a constant which is practically independent of the tempera- 

 ture. In the measurement of self-diffusion it is usually necessary to follow 

 the speed of mixing of two closely related metals, such as gold and 

 silver or tungsten and molybdenum, which is then equated as a first 

 rough approximation to the speed of self-mixing of one component. 

 The velocity of self-diffusion in lead can be determined accurately, 

 without such an uncertainty, by measuring the velocity of diffusion of 

 a radioactive lead isotope in ordinary lead. Pievious experiments of 

 this kind have already been described^. 



This paper will deal with the result of an investigation carried out 

 recently with the object of determining the heat of relaxation of the 

 lead lattice and of ascertaining how far this quantity and the velocity 



^ G. Hevesy and A. Obrutschew.^, Nature 115, (174 (1925). 



