Talk:Perfect crystal

Comments
Is this really a page on perfect crystals, or on crystal imperfections? -- Janet Davis

In order to explain a perfect crystal, I think you should at least mention the attributes that cause a crystal to be imperfect -- [Chris Markides]

We live in a world of flaws or imperfections. A perfect crystal would be composed of atoms (or molecules) that are perfectly aligned. For example a perfect diamond would have all sp3 hybrid carbon atoms in perfect alignment. Since carbon has more than one isotope all natural diamonds are flawed istopoically. I have heard that synthetic diamonds have been made where nearly absolutely pure carbon containing only one isotope was used. The resulting diamond had a hardness exceeding that of natural diamond. Does anyone have references to this? (Bill Scrib)

I'm going to make a new page on Crystallographic Defects, which should resolve this question. -- Olof

How common are perfect crystals in nature? What about manufactured crystals or alloys? -- Janet Davis

Theoretically perfect crystals can only exist at temperatures of Absolute zero or slightly above this. I'm not entirely sure about this though. Logically, since thermal energy causes atoms to vibrate in their atomic positions and eventually move from them (leaving behind a vacancy) this should be the case for metals (and alloys). As for stoichiometric salts, I'm not sure... -- [Chris Markides]

Commercially grown crystals can have no line or plane defects -- i.e. it is possible to completely remove these types of defects from a macroscopic sample. Point defects are another story. Theoretically, there are two cases: if the crystal is not of infinite size, it will have a surface, and a surface is a defect. If a crystal is of infinite size, then there are an infinite number of atomic sites which could have a defect, and ( because absolute zero is an infinitely improbabe state ) there will be at least one which does. Therefore there are no perfect crystals.

Of course, the very idea of perfect crystal is only useful in the theoretical sense of zero entropy / third law of thermodynamics. In materials science there's never a prectical reason to consider the case of zero point defects. -- Olof

I moved the bits about vacancies and interstitials to crystallographic defects

I removed this bit, as it is not necessarily true: sometimes if there are more dislocations, then there are more dislocations which can glide and thus it is more ductile.


 * Linear imperfections are all dislocations and can be simply explained as an extra half-plane of atoms wedged in the lattice. Dislocations are very important to the mechanical properties of a metal as their concentration affects its strength, ductility, and other mechanical properties.  The higher the dislocation density, the stronger and less ductile it is.

I removed this bit, as it is not quite complete: not all planar imperfections are grain boundaries:


 * Planar imperfections are the boundaries between crystals in a material. These are also known as grain boundaries.  The size of grains in a metal also affects its mechanical properties.  The smaller the grain size, the stronger and less ductile it is.