Do FCC Structures only have a CN of 12?

[letshin]

Hi all,

Without first delving too deeply into the literature I wanted to ask if its is only permissible for FCC structures to have a coordination number of 12. In the case of lattice vacancies and/or distortion: wherein the atomic sites are slightly displaced; is there a kind of tolerence for this to happen without breaking the FCC symmetry?

Are there any publications on this topic?

Many thanks,
Let

 

[mfb]

No real crystal is exactly perfect everywhere - you always have some defects. The number of defects is very small compared to the total number of atoms, so usually they can be neglected if you consider the crystal structure.

 

[letshin]

Cheers. When you say small, what is the order of difference?

In the case of a, say, disordered FCC structure how many vacancies would be needed before the structure loses symmetry and say, no longer shows FCC-type diffraction peaks?

 

[mfb]

Cheers. When you say small, what is the order of difference?

That really depends on the crystal, its temperature, purity and so on.

In the case of a, say, disordered FCC structure how many vacancies would be needed before the structure loses symmetry and say, no longer shows FCC-type diffraction peaks?

I'm not sure if such a broken crystal would still be stable enough to use it as a crystal at all.
Reference 2 of the wikipedia article suggests up to one vacancy per 1000 atoms in equilibrium at the melting point of metals. And that is a very high number of vacancies.

 

[letshin]

That really depends on the crystal, its temperature, purity and so on.

I'm not sure if such a broken crystal would still be stable enough to use it as a crystal at all.
Reference 2 of the wikipedia article suggests up to one vacancy per 1000 atoms in equilibrium at the melting point of metals. And that is a very high number of vacancies.

Thanks for that. That helped quite a bit - of course I should have checked wiki first at the very least. Silly me.

 

[Astronuc]

Some background on metals - https://www.nde-ed.org/EducationRes...e/Materials/Structure/metallic_structures.htm

Here is some background on lattice defects - http://people.clarkson.edu/~isuni/Chap-5.pdf

http://research.che.tamu.edu/groups/Seminario/Materials_CHEN313_Spring_2013/HW6-solution.pdf

Some numbers for defects - densities on the order of 1 defect per 10,000 or 100,000 atoms, or less.