Asymmetric Crystal Structure

[The Thing]

I have never really understood what it means. So, what is it? Could anyone provide some info or a link? I've tried googling but haven't found anything.

[RyanJ]

I have never really understood what it means. So, what is it? Could anyone provide some info or a link? I've tried googling but haven't found anything.

 

Umm, as far as I remember nnormlly refers to a Carbon atom has 4 different atoms or structural groups attached to it making an unballanced arrangement (Not a regular lattice).

 

Try this:

 

http://en.wikipedia.org/wiki/Asymmetric

 

Cheers,

 

Ryan Jones

[Tetrahedrite]

Are you talking about chiral compounds?

[The Thing]

Err...not unless chiral compounds exhibit mechanoluminescent behaviors...

This question is one that I had when I was trying to learn about mechanoluminescence, the luminescence of a material from a mechanical stimulus. Only asymmetric crystals exhibit this behavior. So that's my question. What is an asymmetric crystal structure?

[The Thing]

Okay, I've done some research. Assymetric crystals are non-centrosymmetric crystals, lacking a center of symmetry. However, I'm having a difficult time visualizing this. More help please? Does it have to be an irregulary looking shape to have no center of symmetry?

 

The problem is I can't figure out how the atoms will be arranged in the crystal lattice if the crystal has a low symmetry. They wouldn't be stacked neatly like, say, in a cubic or hexagonal crystal, would they? Will it have to have defects to achieve low symmetry?

[BhavinB]

Defects do not cause the non-centrosymmetricity. It is inherent in the basis of the crystal.

 

Definition: A crystal is defined by a set of lattice points and a basis. The lattice points are defined by three vectors. A basis is placed at each point. The basis can be just one atom, two atoms spaced a distance apart, a molecule etc.

 

In the case of a crystal that is not centrosymmetric, the basis also has no inversion symmetry (translating all points to their negative does not give the same basis). Also, if the lattice has no inversion center, then neither does the crystal. Here's a webpage with a picture of the 32 crystal classes. 21 have no inversion center.

 

http://newton.ex.ac.uk/research/qsystems/people/goss/symmetry/Solids.html

 

Interesting to know also that alot of other properties cannot happen in crystals with an inversion center. For example, pyroelectricity and piezoelectricity. Also, a crystal with an inversion center cannot have second order non-linear optical properties (if that means anything to you).

[The Thing]

Thank you thank you. That was EXACTLY what I needed BhavinB.

And what is a second order nonlinear optical property? It doesn't mean anything to me, but I'd like to know.

 

Incidentally, would you know anything about mechanoluminescence? Why do crystals with impurity defects in lattice also exhibit mechanoluminescence?

[BhavinB]

second-order nonlinear optical property means the materials refractive index varies with light intensity. Usually we believe the refractive index is a constant, but at sufficiently high intensities, it isn't.

 

so n=n0 + n2*I

where I is the intensity of light.

 

I don't know anything about mechanoluminescence. You'll have to describe the property.