Graphite vs Diamond

Neither of these is an element. Both are carbon — the same atoms, in the same isotopic mix, differing only in how they are joined. Graphite and diamond are the textbook case for the idea that in a solid, arrangement matters as much as composition, because these two arrangements produce the softest common mineral and the hardest known natural one out of exactly the same ingredient.

Two geometries, and everything follows from them

In graphite each carbon bonds to three neighbours in a flat hexagonal sheet, leaving one electron per atom in a delocalised cloud above and below the plane. The sheets stack about 335 picometres apart and are held to each other only by dispersion forces — far weaker than the bonds inside each sheet. Graphite is therefore strong in two dimensions and barely held together in the third.

In diamond each carbon bonds to four neighbours at the corners of a tetrahedron, and the bonding runs continuously in every direction. There is no plane of weakness and no spare electron. The whole crystal is one molecule.

Every difference between them is downstream of that. Hardness, conductivity, colour, density and market price all trace back to whether the fourth electron is in a bond or in a cloud.

Graphite is the stable one

Under ordinary conditions diamond is not the stable form of carbon. Graphite is, by about 1.9 kilojoules per mole — so every diamond in existence is slowly, thermodynamically, in the wrong phase. The energy barrier to rearranging the lattice is so enormous that the conversion takes geological time even at elevated temperature, which is why this is a fact about thermodynamics rather than a warning about jewellery.

Pressure reverses the preference, because diamond packs the same atoms into a considerably smaller volume: 3.51 grams per cubic centimetre against graphite's 2.27. Squeeze carbon hard enough and the denser phase wins. Industrial synthesis exploits exactly that, at pressures of several gigapascals and temperatures well over a thousand degrees with a molten metal solvent to speed the rearrangement. General Electric announced the first reproducible synthesis in 1955; a team at ASEA in Sweden had almost certainly done it two years earlier but kept the result unpublished, and the priority question has never been cleanly settled. Chemical vapour deposition later found a route that ignores pressure altogether, growing diamond film atom by atom under conditions where graphite is the stable phase and simply not letting it form.

Two ways to move heat, one way to move charge

No bulk material moves heat as well as diamond does at room temperature; it outruns copper several times over, and it manages this with no free electrons at all. The transport is purely vibrational: a stiff, light, perfectly regular lattice carries phonons superbly. That is why diamond heat spreaders appear under high-power laser diodes and radar amplifiers, where the problem is getting heat out of a very small area very fast.

Electrically the two could hardly differ more. Diamond's forbidden gap is around 5.5 electronvolts, wide enough to make it a fine insulator and to leave it transparent far into the ultraviolet. Graphite's delocalised electrons make it a conductor along the sheets and a poor one across them, an anisotropy of roughly a thousandfold in the same crystal. Industrial graphite exploits both sides: electric arc furnace electrodes need the conduction, and graphite gaskets and crucibles need the refractoriness.

Doping complicates the neat story. Boron-doped diamond is a working semiconductor, and at heavy doping it becomes superconducting at a few kelvin — an insulator turned superconductor without changing the lattice.

The lubricant that stops working in space

Graphite's slipperiness is usually explained as sheets sliding over one another, and that explanation is incomplete in a way that mattered.

Dry graphite in vacuum is not a lubricant. It is an abrasive. The low friction depends on water vapour and other gases adsorbed between the layers, which is fine at sea level and fails at altitude. Aircraft electrical machines in the Second World War suffered rapid brush wear at high altitude for precisely this reason, and the fix was to add compounds that restored the film. Spacecraft mechanisms use molybdenum disulfide instead, which does not need the atmosphere.

Diamond, meanwhile, has genuinely low friction against most materials in most environments and is used as a hard, low-wear coating on tooling — the opposite trade.

Lavoisier's burning glass

That diamond is carbon was not obvious and had to be proved. In 1772 Antoine Lavoisier used a large lens to focus sunlight on a diamond in a sealed vessel and showed that the gas produced was the same fixed air that burning charcoal produced. Diamond burns, at temperatures a few hundred degrees above where graphite starts oxidising in air. Both are combustible; only one of them is sold in a ring box.

There is a pleasant footnote on the graphite side. The material in a pencil was mined at Borrowdale in Cumbria from the 1560s and mistaken for a lead ore, which is why the word "lead" has clung to pencils for four and a half centuries despite there being none in them. The hardness grades on a pencil are set by the clay-to-graphite ratio, not by the graphite.

What each is bought as

Graphite arrives in two commercial forms with different supply stories: natural flake graphite, which is mined and then processed, and synthetic graphite, made by baking petroleum coke at graphitising temperatures. Between them they go into steelmaking electrodes, refractories, brake linings, and — the fastest-growing sink by far — the anodes of lithium-ion cells. Natural graphite processing is heavily concentrated in a single country, which has put it on critical raw material lists in both Europe and North America.

Diamond splits even more sharply. The overwhelming majority of industrial diamond by weight is synthetic grit costing very little per carat, used in saw segments, grinding wheels, wire-drawing dies and dressing tools. The gem trade is a separate market with separate economics, and laboratory-grown gem material has pulled the price of that side apart over the last decade.

Which arrangement of carbon for which job

• Cut, grind or drill something hard — diamond, in grit or as a coating.
• Carry a very large current into a furnace — graphite.
• Get heat out of a millimetre-square hot spot — diamond.
• Store lithium ions — graphite.
• Lubricate something that will operate in vacuum — neither; graphite will fail there.
• Make an optical window that must survive a high-power beam — diamond.
• Withstand a furnace atmosphere without melting — graphite, which sublimes rather than melts and holds its strength as it heats.