BCC Metals: An Anomaly in Atomic Packing Factor?

[Raziel2701]

If FCC and HCP are the ones with the highest atomic packing factor, why would there be metals with BCC structures?

 

[Vanadium 50]

Why does nature want a high packing factor?

 

[Dickfore]

It seems that all the metals that have a bcc structure are transition metals. I list them here, together with their electron configuration, and their magnetic ordering:

1. ₂₆Fe [Ar] 3d⁶ 4s², ferromagnetic
2. ₂₄Cr [Ar] 4s¹ 3d⁵, antiferromagnetic (SDW)
3. ₇₄W [Xe] 4f¹⁴ 5d⁴ 6s², paramagnetic
4. ₄₁Nb [Kr] 4d⁴ 5s¹, paramagnetic

I mention magnetic ordering, because I saw iron among them, and whenever I see iron, I think of ferromagnetism. Also, notice that the magnetic transition temperature (1043 K) is very close to a structural phase transition (fcc -> bcc, bcc existing at lower temperatures) temperature 1185 K.

I would speculate that the exchange interaction, usually responsible for magnetic ordering, plays a significant role in these metals, and lowers the energy in a bcc structure, than an fcc structure.

 

[Raziel2701]

So the reason for the existence of BCC metals has to do with magnetic characteristics? Ok that is definitely a good thing to think about. Unfortunately I don't know much about those. In any case, thanks for showing me this.

 

[Astronuc]

So the reason for the existence of BCC metals has to do with magnetic characteristics? Ok that is definitely a good thing to think about. Unfortunately I don't know much about those. In any case, thanks for showing me this.

Not magnetic characteristics, but probably electron configuration.

http://en.wikipedia.org/wiki/Periodic_table_(crystal_structure)#Table

The bcc atoms tend to have higher strength and higher melting temperatures than others in their respective periods.

 

[Studiot]

It seems that all the metals that have a bcc structure are transition metals.

Actually the alkali metals (group1) and barium and radium(group2) also have a BCC structure.

I echo Vanadium's comment and further ask is the packing difference (68 as opposed to 74%) so very large?

 

[DrDu]

According to conventional wisdom, BCC crystal structure in these elements is due to directed covalent bonds involving the d-orbitals.

 

[Enthalpy]

Ferromagnetism is not a property of the elements. It's a molecular property.

Even in a metal, the bonds depend on the underlying electronic shells. That's why diamond, silicon and germanium, which have as many valence electrons and the same crystal structure, have different conduction band structure.

So a reason can be: because the preferred bonds of the element influence the crystal structure. In other words, metallic bonds are not just a matter of packing spheres in a sea of electrons.

You spoke about pure metals, didn't you? Because in alloys, different atom diameters can favour varied crystals.

 

[DrDu]

Actually the alkali metals (group1) and barium and radium(group2) also have a BCC structure.

I echo Vanadium's comment and further ask is the packing difference (68 as opposed to 74%) so very large?

Correct.
The alkali metals are discussed in many texts on solid state theory like Ashcroft and Mermin.
You also have to take in mind that in BCC, an atom has 8+6 nearest neighbours while in the closest packed structure only 12.