Hexagonal Diamond - The "other" form of diamond

Here's another geometrical object that, if you believed basic school textbooks, should be impossible. You know how they taught you that carbon only comes in three forms, diamond, graphite and soot, and that other configurations were geometrically impossible? Before the penny dropped regarding Buckyballs and Buckytubes?

Well, it turns out that even diamond has (at least) two possible versions.

This one is known as hexagonal diamond, or Lonsdaleite, after the crystallographer Kathleen Lonsdale (1903-1971).

The reason why most people haven't heard of it is that it's not normally naturally occurring, at least, not in situations that are easily accessible to us (although teeny-tiny specks of it are supposed to have been isolated from meteorites). Its nominal bond angles and lengths would seem to be the same as normal diamond, and it still has a tetrahedral aspect to the way that it has four bonds surrounding each individual atom, but the configuration is, nevertheless, different. People have computer-modelled Lonsdaleite, but I only know of two physical models of the structure, and they're both in my bedroom. *

Hexagonal diamond is a bit of a wildcard, in that although we can try to calculate and model the properties of the bulk material, we don't really know for certain what they are, exactly. We expect pure Lonsdaleite to be harder than standard diamond (which is interesting), and it it might well have interesting semiconductor properties when we dope it (as with normal diamond), but until we can find or make a decent-sized chunk of the stuff to test, we don't know for sure.

What we could do is try to make hexagonal diamond using conventional chemical vapour deposition (CVD), but to use some sort of crystal seed surface that has bumps and hollows in the right places to get the Lonsdaleite structure started, after which the deposited film will hopefully continue growing in the new "HexD" configuration. But it's maybe not immediately obvious why Lonsdaleite doesn't usually get noticed in normal diamond-bearing rock. Does it have some form of instability that makes it a less viable end-material than conventional "cubic" diamond? Dunno.

One potential clue is Lonsdaleite's structural affinity to graphite. You can (notionally) make Lonsdaleite by taking stacked and aligned sheets of graphite and cross-linking. Graphite's two-dimensional sheets only make three out of the four potential bonds per atom, the missing fourth bond being shared as a sort of pair of fuzzy electron clouds that hover on both sides of each individual graphene sheet, as a sort of repulsive lubricant that lets the individual graphene sheets slide across each other. Looking at the hexagonal structure of a single graphene sheet, we can select alternating carbon atoms (three per hexagon), and force them down out of the plane to make bonds with the corresponding atoms on the sheet below … and then take the other 50% of the atoms in the sheet and make them form similar shared bonds with the atoms directly above them, in the next sheet up. The sheet then crinkles so that it's no longer flat, and hopefully, in the right set of circumstances, the sheets on either side will start to crinkle to fit, and their spare three-bond atoms will be pushed out of the plane to be closer to the next sheets, and will hopefully start to make bonds.

So, perhaps we could try making Lonsdaleite by clamping the edges of a block of graphite to compress the constituent graphene sheets and encourage them to "crinkle", while ... er ... heating? Or repeatedly hitting the thing with a hammer? That might create disordered Lonsdaleite, which might have pockets or regions of "the good stuff", which might then be extractable. And even if it doesn't work, it might produce something interesting, maybe. What the heck, why not go for the whole "Frankenstein laboratory" approach and try zapping a current across the sheets at the same time, to see if you can encourage something interesting to "grow". :)

But perhaps chemical vapour deposition is the way to go, if we can find a suitable seed substrate.


* Okay, somebody's now bought one though my Shapeways shop, so that makes three. :) There must be other physical models of this thing out there, in chemistry labs somewhere ... I've just not seen one photographed. Then again, I'm not a chemist or a crystallographer.

Holographic Diamonds

Back in January 2000, the Millennium Dome exhibition opened to what was supposed to be a display of the best of British achievement. Unfortunately the people in charge of setting it up didn't seem to have a clue how to run this sort of exhibition or what to put inside the dome, and it ended up as a bit of a national embarrassment.

One of the last-minute additions to the show was the Millennium Star diamond.

To see the diamond, you had to walk though an angled passageway that was completely pitch black apart from some slightly odd (monochromatic?) blue light, and there, in the middle, you'd see a case walled with bulletproof glass, containing the blue-lit diamond. You walk past it, perhaps pause, and then make your way out. No loitering, no photography.

Something struck me when I was in there. The thickness of the cabinet's glass meant that the diamond appeared be in different places, depending on which pane you viewed it through - that's completely normal, you usually see a similar effect with fishtanks. But the blue light confused me, because normally you only see blue-lit rooms when someone's trying to hide something. Okay, so it was a blue diamond, but still ...
The human eye is pretty bad at seeing sharp details in blue light, which is why Windows has traditionally had a blue-themed startup screen - the old splash screen used crude dithering to recreate the effect of a smooth variation in tone using the default 16-colour VGA pallette, and by doing this in blue, the eye was fooled into not noticing the effect too much. If Windows 3.1 had tried that trick in red or green or yellow, the result would have been bitty and grainy and would have looked awful. In blue, you can't see the fine detail that gives the trick away.

Now, the glass.
Bulletproof glass uses a "sandwich" of alternating toughened glass and shock-absorbing plastic sheets, so that even if you shatter every layer of glass, the shatter-patterns are different, and the pieces stay stuck together by the plastic. If someone had simply added an additional sheet of plastic film with a with a hologram of a diamond ... then how would you be able to tell? You couldn't look for alignment errors between the sheets on different panes, because the diamond woudl appear at differtent positions when viewed through the different panes anyway, due the the thickness of the glass.
Does a holographic diamond appear to refract light in the same way as a real diamond? I don't know, but if someone wanted to look for an "anomalous" spectrum effect that didn't correspond to real diamond, the use of monochromatic blue light might be a good way to stop them. And with single-colour light source, we'd also find it difficult to see any interference fringes due to misregistration of the holographic films. Optical theory says that to see those coloured fringes, the colours already have to be present in the original lightsource, andf in our "blue room", that light wouldn't be there.

Of course, for all this to work, de Beers would have to have their own in-house holography R&D department aligned with their security people, which sounds pretty unlikely. But in fact, deBeers do have very strong links to holographic reseach: They have laser systems for checking diamonds, and for laser-etching holographic security marks onto them, and slightly more peripherally, Lucent have been researching diamond as a potential holographic storage medium. DeBeers also have a holographic diamond passport scheme. So diamonds and security and holography research and lasers and de Beers all have a pretty strong overlap. There probably aren't that many companies that know more about certain sorts of holography than de Beers do.


So here's a fun, harmless little conspiracy theory to ponder that's worthy of Sherlock Holmes or Jonathan Creek: What if this diamond, which thieves tried to steal from the Dome in November 2000 in a ram-raid using a mechanical digger, nailguns and a getaway speedboat, was protected by the ultimate "stage magic-based" security system? What if the diamond, that perhaps many thousands of people would swear on oath to having seen in person ...
... was never actually there?

Physics Fraud, and the Impossible Diamond

Physicists used to tell me was that physics was a special subject, because you never had to worry about the possibility of fraud. Their reasoning was that You Can't Fake Physics. If you make up an experimental result that isn't right, you're doomed to be found out when other people try the same experiment and can't replicate your result. It's a dumb thing to do, and no physicist would ever be stupid enough to try.

However, it might be more accurate to say that perhaps no sane physicist would try to fake a result that they believed to be wrong. Faking a correct result may be cheating, but doesn't carry the same risk. It's much more difficult to spot a fake result when it agrees with everyone else's results and with what everybody expects to happen.

We can sometimes spot a "false positive" when a theoretical prediction that is successfully verified later turns out to be wrong, or when an experimental technique later turns out to be impossible, or impossible to conduct to the claimed accuracy. When this happens in an experiment that contradicts current theory, we usually rip the person responsible to shreds, and accusations start flying of scientific fraud. When it happens in an experiment that agrees with current theory, we're usually more charitable, and tend to say that perhaps the experimenter was simply mistaken, or overcome with a little too much enthusiasm. There's such a large grey area for honest mistakes, or the unconscious selection of "good" data (or simple wishful thinking) that a certain amount of bad science probably slips under the radar without being spotted, and it's not often that we find a "bad" result supporting a "good" outcome that's really so profoundly impossible that people are forced to consider using the "f" word.

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One candidate case happened in 1955.
Researchers had been wanting to create artificial diamonds since at least as far back as Nineteenth Century. When H.G. Wells published his short story "The Diamond Maker" in 1894, a number of researchers had already been trying approaches with varying degrees of optimism and claiming positive results, including James Ballantyne Hannay in 1880, and Nobel Prize-winner Henri Moissan (also in 1894). One of the wildest attempts to create artificial diamond was carried out by John Logie Baird, who briefly blacked out of part of Glasgow when he deliberately short-circuited an electricity substation's power terminals across a graphite rod embedded in reinforced concrete (the story goes that he couldn't work out how to get the thing open afterwards, and it ended up at the bottom of a river, unexamined).

The potential financial payoff for anyone able to create artificial diamonds on demand was obvious, and by the 1950's there had been more reported (but often disputed) successes, and competing researchers were trying desperately hard to be the first people to produce a proper, replicable, accepted process that definitely did produce diamonds. One team in particular figured that they were on the edge of actually achieving it. They had the theory right, they had the equipment right ... the only problem was that their pressure-vessel obstinately refused to cough up any diamonds.
It was desperately unfair. They'd done all the work correctly, and the experiment refused to come out the way it was supposed to. They needed a diamond to get further funding. From their perspective, they probably reckoned that they deserved a diamond. It was necessary for their future research. Science needed a diamond!

And a diamond dutifully appeared. They got new funding, bought new equipment and replicated the result, others managed the same thing, and everyone was happy.

Except that ... someone went back and checked the calibration on the original pressure reactor and found that its readings had been significantly "off". The pressure-vessel had been running at too low a pressure for diamond to form. With hindsight, their original artificial diamond seemed to have been a physical impossibility. So how did it get there?

Three of the four original team members put their names to a letter to Nature in 1993, explaining that subsequent spectral analysis of the "run 151" diamond years later had shown that it appeared to have the characteristics of a natural gemstone rather than those of an artificial rock. The experimenters had carried a small stock of natural diamonds for research purposes, and it seemed that one of those had somehow found its way into the pressure vessel during setup, and been "fortuitously" discovered after the experiment.

It's quite a nicely- and elegantly-written letter, but the authors must have been acutely aware that to most people, the idea that one might "accidentally" lose a real diamond inside an apparatus designed to create artificial diamond, in such a way that it could then be rediscovered and used to get further desperately-needed money ... if this happened in any other field, we'd tend to assume deliberate fraud.

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Another thing that might surprise some outsiders is that although the announcement that the experiment had been a success was made in 1955, the retraction didn't happen until 1993, nearly forty years later. For Twentieth-Century experimental physics, this wasn't actually all that unusual – there seemed to be an unspoken "gentlemen's agreement" that if someone had claimed a "correct" result that they shouldn't have, that the community would hold off making too many pointed suggestions in print until some time after the person concerned was safely dead. This was probably a great way of avoiding public controversies, but it also meant that we never really got to the bottom of what had happened in many of these cases. If you weren't supposed to go public while someone was still alive, but you couldn't suggest fraud after they were dead (because it was unfair to level that sort of accusation at someone when they couldn't defend themselves), then it meant that anyone who did get up to no good had a decent chance of not being publicly outed, in print, ever. By the time a critical report could be written, the people with first-hand knowledge of what had really happened might have all died off.

By avoiding investigating these cases until after it was too late to reach a conclusion, the physics community probably did manage to achieve a nominal "no confirmed mainstream fraud" result. But that result was itself not especially honest.

Things are now looking up. Berkeley recently went public very quickly about problems with the work of two physicists (in two separate cases) who seemed to have been almost routinely fabricating data to get their "world-class" results (Victor Ninov and Jan Hendrik Schön), and there've now been a few more speedy "outings" of scientists caught misbehaving. So from now onwards, the more temptation-prone members of the physics community know that if they gain fame and fortune by faking data, universities and comissioning bodies won't necessarily hush the thing up for them.

But for research published before 2000 (or perhaps before ~2005) ... be more careful. A certain number of the "jewels" in physics history aren't quite what they appear to be.

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• "Errors in Diamond Synthesis", Nature 365 2nd Sept 1993, p.19