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Cooperative positional disordering
leading to melting

Contemporary models of bulk melting consider positional disordering as a cooperative defect, involving a number of atoms. A characteristic feature of cooperative defects is that the energy of their formation can be greatly reduced relative to isolated point defects. However, this kind of crystal disorder is subject to the condition that the appropriate sequence of neighboring sites must be displaced cooperatively from the ideal lattice positions. Within recent years much work as been done on a particular kind of cooperative defects known as lattice dislocations. As is well-known, a point defect consists either of a missing atom in the lattice or an extra atom between two normal lattice points. If more than one adjacent point defect occurs in a crystal here may be a slip along a surface causing a line defect which is called a dislocation (See Fig. 1.8).
Figure 1.8: Dislocation is a line defect.
\begin{figure}\centerline{\epsfxsize=3.5 cm \epsfbox{/home/phsorkin/Diploma/Theory/Pict/crysd.eps } }\end{figure}
Dislocations also contribute to melting of metals. Theories of dislocation-mediated melting have several appealing features; the free energy of a crystal containing a dense array of dislocations is comparable to the free energy of its melt. The fluidity of the melt can be attributed to the mobility of dense array of dislocations. The part of the total energy of a solid, saturated with dislocations, attributed to these defects is comparable to the latent heat of fusion. The presence of dislocations lowers the energy of creation of additional dislocations; if this reduction is sufficiently strong it can lead to an avalanche of dislocations in a first-order transition. A number of experiments and computer simulations have given some support for these theories of dislocation induced melting, but the evidence is not clear enough to conclude that dislocation-mediated melting has been demonstrated.


next up previous
Next: Surface melting Up: Bulk melting Previous: Melting theories based on
2003-01-15