Crystal Structure & Metallic Bond: What Comes First?

[Raziel2701]

For metals for instance, is the metallic bond that determines the crystal structure(i.e. FCC (face centered cubic), HCP (hexagonal close packed)) or is the crystal structure that determines the type of bond?

In other words, if someone were to ask me what kind of bond an element with an FCC structure has, is that all I need to know to figure it out?

 

[chill_factor]

For metals for instance, is the metallic bond that determines the crystal structure(i.e. FCC (face centered cubic), HCP (hexagonal close packed)) or is the crystal structure that determines the type of bond?

In other words, if someone were to ask me what kind of bond an element with an FCC structure has, is that all I need to know to figure it out?

crystals don't necessarily have to be FCC or HCP, different types of bonds can have both (ionic and metallic; you can't say that "oh this is FCC it can't be metallic/ionic").

in short the specific type of intermolecular force has little to do with the crystal structure.

 

[DrDu]

In general this is a very difficult question. For simple metallic alineations like brass, the structure depends on the electron density, look for Hume Rothery phases.

 

[JohnRC]

In general this is a very difficult question. For simple metallic alineations like brass, the structure depends on the electron density, look for Hume Rothery phases.

DrDu is quite right. Here is an interesting case for the original poster to consider: Cobalt and nickel have near identical molar masses, near identical atom sizes, and exactly identical metal densities. But cobalt packs as hexagonal close-packed, while nickel packs as cubic close-packed (i.e. face-centred cubic). Both metals have very similar chemistry – there is some very subtle factor in the bonding that tips the balance between the two close-packed structures.

 

[snorkack]

I´d say that it is both ways.

Consider carbon.

Carbon is able to have either sp3 bonds, in saturated compounds and in diamond, or sp2 bonds, in unsaturated compounds and in graphite.

The physical environment, neighbouring atoms around the carbon in molecule and cristal, can and do force the atom to have either sp3 or sp2 bonds.

But one or other of these makes for a poorer fit. And it is the good or poor fit of atoms and their bonds into molecule or crystal structures that drives chemical reactions and allotropic transition.

In case of carbon, diamond is very slightly a poorer fit than graphite. But the bonds are still so strong that breaking them to rearrange then for better fit takes heating over 1000 celsius.

Now have a look at cobalt and nickel, and iron for a good measure.

Iron can fit together either as body centered cubic or face centered cubic.

These 2 are actually so close in energy that on heating, BCC iron converts into FCC at 912 degrees - and then back into BCC at 1390 degrees.

(Iron can also have HCP ε-Fe at high pressures above 100 kbar).

Now, neither cobalt nor nickel form BCC. Both are FCC at high temperatures - and freely mix with FCC iron.

Addition of nickel (FCC at all temperatures) to iron stabilizes FCC iron, because nickel mixes freely with FCC iron, but makes a poor fit in BCC iron. Up to about 6 % of Ni does fit into BCC iron, but not more.

If over 20 % of Ni is added to FCC iron, then the resulting FCC structure is stable at all temperatures, and is used for stainless steel.

If, however, the nickel concentration is between 6 % and 20 % then on cooling, there is not enough nickel to stabilize FCC, but too much to fit in BCC. The result is that BCC crystallizes out in separate crystals which reject nickel in growth - forming Widmanstätten pattern.

Now, pure cobalt is FCC above 420 degrees - and forms HCP below 420 degrees. But since at such a low temperature allotropic transitions are hard, the transformation often does not happen or happens only partially.

But unlike Ni, Co fits fairly well into BCC, so that Fe-Co alloys up to about 70 % Co transform to BCC at lower temperatures. The alloys closer to Co stay FCC unless they are over 90 % Co and can carry Fe into HCP structure.

There have been predictions that Ni should also be able to form HCP structure - only below about 200 degrees. But since transformations would not happen at these temperatures, HCP nickel has never been found.