I was thinking about about x-ray crystallography and how it can be also used to determine crystal defects.
After all, real crystals are not perfect- there is usually some regions where the arrangement of the ions differ. Examples include point defects-consisting of missing or additional ions or impurities and line defects-also known as dislocations whereby the some of the atoms in the lattice are misaligned. Crystal defects are interesting cause they can significantly change the properties of the crystal. One of the obvious effects is the electrical conductivity of the material. Misalignment of the atoms in the lattice will reduce the strength of the crystal to external forces. Reading up on this topic, in some cases, crystal defects can also change the colour of the material. When an negative ion vacancy is filled by an electron, the localized electron can absorb light in the visible spectrum such that the otherwise transparent perfect crystal becomes coloured.
Out of curisoity, why is x-ray crystallography so popular? You guys (point out Josh specfically) seem to mentione it freuqently?
ReplyDeleteI like the idea of impurities in crystals creating colours...makes jewellery stores so sparkly and colourful!
Crstal defects, defects maybe, but really does give you an appreciation of shiny things...
Well, x-ray crystallography allows you to directly measure the inter-atomic structure of your crystal. That is, it allows one to observe where, in space, your atoms are in relation to each other. I think this was the first (correct me if I'm wrong...) type of measurement of this kind, allowing scientists an amazingly clear picture of the atomic structure of crystals. cool yeah?
ReplyDeleteIt's funny that the most valuable jewels are colourless though... perhaps mostly coincidentally since big, regular diamonds are hard to find.
ReplyDeleteI would defend my mentioning of x-ray crystallography since for a long time it was the only way to get even a theoretical picture of the potential of a crystal (pre-MATLAB, of course :) ). Consequently, we are currently studying it quite thoroughly in Condensed Matter... :). But if you prefer, we can talk about exchange electron paramagnetic resonance spectroscopy instead. I would find that interesting since I'm doing it for my project :p.
I think that x-ray crystallography is still one of the most used techniques for imaging on the atomic scale (with neutron, human and electron diffraction the other obvious examples): if you can't crystallise a molecule, you can't do crystallography on it and so you can't find out about its geometric structre very easily! (remembering that NMR and EPR don't give any actual images). This is why people do molecular dynamics models of lipid bilayers (like me) and get squillons more dollars than your average physics department to build big (and not necessarily necessary) supercomputers. Besides, electrons are more interesting than nuclei for chemistry, so x-rays are usually more well suited to a given set of circumstances.
I think too, Dave, that x-ray crystallography was first, mainly because of the ease in creating and using x-rays compared with electrons (hard to uniformly generate) and neutrons (hard to do without nuclear reactors :( ). It's pretty cool, especially considering that the father and son team that did this were Australian, and that Bragg the Younger was the youngest Nobel prize winner at the time (perhaps youngest ever?).
Joseph, perhaps the effects on conductivity/properties (etc.) of impurities are easier to study than the x-ray diffraction pattern? After all, you get an average picture of the crystal, so to see a defect, you would need a systematic defect so the averages would all add up. Although you could measure the resolution to investigate this.... Also, some of the macro-defects that generate pretty crystals can be viewed using microscopes, especially if they're thin (so you might be able to slice a crystal and electron microscope it).
Yeah, seeing the inter-atomic structure does seem pretty cool, and would be quite significant particularly if you were the first to show it!
ReplyDeleteThanks for the clear up.
It's pretty recent and one must appreciate how this field has grown so rapidly.
X-rays are from just before the 1900s. Before then, would have no hope of seeing interatomic structures.
Yet (not even) 100 years later, we (students in general) are learning about interatomic structures in high school, say chemistry.
Josh, looking around here, here and here, one would conclude that coloured rocks are more valuable, notably, coloured diamonds...
How is the going exchange electron paramagnetic resonance spectroscopy, by the way?