Ask The Naked Scientists

Ask the Naked Scientists SA episode

Fri, 6th Sep 2013

Do Diamonds Melt?

A slightly misshapen octahedral diamond crystal in matrix. (c) USGS

How do robotic surgeons work? Do diamonds melt? What is heartburn? Can fish see colours, and what should anglers wear? What is brain plasticity? What would happen if an egg were dropped into boiling oil? What causes blushing? Why does holding the antenna of an AM radio improve reception?

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Good questions. Diamond is a network solid and melting has no meaning for it. I mean there are no constituent atoms/molecule but only one big molecule of carbons atoms. Since the C-C bonds are very strong, it requires very high energy to break them and if it occurs; that is no longer diamond but gaseous carbon atoms.

What do you say? adianadiadi, Mon, 7th Oct 2013

"Diamond is a network solid and melting has no meaning for it. "
Does ice melt?

This page
Suggests that if the pressure is about 100,000 atmospheres and the temperature is over 5000 degrees then yes, diamonds melt Bored chemist, Mon, 7th Oct 2013

Interesting; thanks for the reference bored chemist. chris, Wed, 9th Oct 2013

Ok, I've read that and I am quite surprised; I'd always thought that diamond did not melt. That's a learning point for me for today! chris, Wed, 9th Oct 2013

I agree that at the right temperature and pressure diamond (or almost anything) can be transformed into a liquid, but I'm not sure if I would call it melting. Certainly the liquid would not be diamond, just some form of liquid carbon, even though it could return to diamond form if cooled at the right rate.

Phase changes of lattice materials are also chemical changes. Molecular substances can melt without breaking any covalent bonds, so the chemical properties of the solid and liquid are generally close enough we can call it the same substance. I would not call water ice a lattice material--it is still a molecular solid, although it is about as close to a lattice material as can be achieved by a molecular solid. O-H bonds within water molecules are ~0.10 nm while the H--O distances between water molecules in ice are ~0.18 nm, which is consistent with a "hydrogen bond" which is somewhere between a covalent bond and a very strong dipole-dipole interaction. chiralSPO, Wed, 9th Oct 2013

" Certainly the liquid would not be diamond, just some form of liquid carbon, even though it could return to diamond form if cooled at the right rate."
It would be molten diamond in exactly the same was that water is molten ice.

" I would not call water ice a lattice material"

Others disagree

Though, within that lattice, they can rotate.

It's a matter of definition, and one that doesn't seem to help much.
If I heat ice, it melts if I heat diamond it turns to a liquid but for some reason, you don't want to call that melting.

(The requirement "under enough pressure" applies to both cases too).

Bored chemist, Wed, 9th Oct 2013

If, at a specific pressure and temperature, diamond becomes liquid in a condition of thermodynamic aequilibrium (as you say, if cooling the liquid a little it becomes a diamond again) then it's exactly "melting". If it's not an aequilibrium condition and you can't reverse the conditions to obtain a diamond again, it's not properly a melting.

lightarrow lightarrow, Fri, 11th Oct 2013

Does Graphite melt? its the same question.. and the premise of liquid diamond and liquid graphite being different would be interesting to understand.

They would certainly burn, (given enough temperature to break the covalent bonds), but surely they would sublime other than that (similar to CO2)
SimpleEngineer, Thu, 17th Oct 2013

Does Graphite melt?
According to that phase diagram yes, at rather lower pressures than diamond. Bored chemist, Thu, 17th Oct 2013

The main components of the diamond is carbon, is non-metallic elements.
After the high temperature it will only become carbon dioxide. lucas2, Mon, 25th Nov 2013

Diamonds will burn if  there's air present, but you can remove the air and replace it by something less reactive easily enough.
Bored chemist, Mon, 25th Nov 2013

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