Do FCC Structures only have a CN of 12?

In summary, the conversation discussed the permissible coordination number for FCC structures and whether there is a tolerance for lattice vacancies and distortions without breaking the FCC symmetry. It was also mentioned that real crystals always have some defects, but they are usually negligible compared to the total number of atoms. The number of vacancies needed to break the symmetry and lose FCC-type diffraction peaks varies depending on factors such as crystal type, temperature, and purity. Reference 2 of the Wikipedia article suggests that at the melting point of metals, there can be up to one vacancy per 1000 atoms in equilibrium. Some background on metals and lattice defects was also provided for further reading.
  • #1
letshin
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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
 
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  • #2
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.
 
  • #3
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?
 
  • #4
letshin said:
Cheers. When you say small, what is the order of difference?
That really depends on the crystal, its temperature, purity and so on.

letshin said:
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.
 
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  • #5
mfb said:
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.
 
  • #6

1. What is a FCC structure?

A FCC (face-centered cubic) structure is a type of crystal structure in which the atoms are arranged in a cubic lattice with additional atoms located at the center of each cube face. This results in a structure with a high degree of symmetry and close packing of atoms.

2. How is the coordination number (CN) determined in a FCC structure?

The coordination number in a FCC structure is determined by counting the number of atoms that directly touch a central atom. In this case, the CN is 12 because each atom is in contact with 12 other atoms.

3. Do all FCC structures have a CN of 12?

Yes, all FCC structures have a CN of 12. This is a defining characteristic of this type of crystal structure.

4. Are there any exceptions to the CN of 12 in FCC structures?

No, there are no exceptions to the CN of 12 in FCC structures. The high degree of symmetry and close packing of atoms in this type of structure results in a CN of 12 for all atoms.

5. Why is the CN of 12 important in FCC structures?

The CN of 12 is important in FCC structures because it affects the physical and chemical properties of the material. For example, materials with a high CN tend to have high melting points and are usually strong and dense.

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