chapter  37
5 Pages

Motions in three aggregate states

The result obtained on the impossibility of achieving zero temperatures in real conditions does not contradict the Nernst heat theorem. As T tends to zero θV also tends to zero, and in the lowtemperature limit the equilibrium defect crystal must be freed of vacancies. This expression is obtained from the analysis of statistical sums, in which the number of configurations realized for all possible arrangements of atoms and vacancies along lattice sites is counted [8-13]. Each of the configurations is realized by permuting atoms and vacancies, but the real trajectory of their displacements is not concretized, just as the time necessary for such a rearrangement is not concretized. Since there are no restrictions on the way of enumeration of all configurations, the final result agrees with the Nernst theorem. In the actual process of lowering the temperature in a single crystal, during its relaxation, there is a single diffusion mechanism for the redistribution of atoms and vacancies. It can be shown [33] that the limiting result of such a process at infinite times leads to the same expression for the vacancy density, as well as the direct calculation of all configurations. However, the relaxation times become so large (see Figure 36.1 and the data in Table 36.2) that during the experiment for about ten years such states at low temperatures are not realized.