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Saturday, April 21, 2012

How does particular metal crystallize to particular structure?

The reason for a particular metal to crystallize in a particular structure must be sought in the fact that the free energy E-TS for this system is lower than that for any other structure.

The same remark may be made with reference to those metals which have different structures in different temperature regions(Allotropy). This phenomenon is exhibited especially by three and four valent metals and by the transition metals.

For instance, Alpha Iron(BCC) is stable upto 910 deg.centigrade; between 910  and 1400 degree Centigrade the stable structure is gamma Iron(FCC); between 1400 and the melting point (1530) the structure is again BCC( Delta iron).

The transformation from one structure to another is dictated by the requirement of minimum free energy. This does not mean that such transformations takes place as soon as the existing structure becomes unstable.

In fact, a transformation of structure involves a rearrangement of atoms, and such a process may take a long time. The reason is that even though the free energy after the transformation is lower than in initial state, the two states are usually separated by an energy barrier or activation energy.

Thermodynamics specifies only the equilibrium condition but doesn't give any information about the velocity of reaction or processes involved in establishing equilibrium. From the atomic point of view, the stability of crystal structures is a problem of cohesive energy, involving the interaction between the atoms.