Powder XRD FCC spirals explanation

[mohammed.omar]

Hello All,

I've been reading about XRD and found this picture online:
http://en.wikipedia.org/wiki/File:Aluminumpowderpattern.png

My question, which might be due to the lack of background as I'm an ECE guy, is why are the FCC lattice vectors depicted of Spirals whose minima lie on the vertical axis?

 

[M Quack]

The spirals are calculated to help find the lattice parameter of the sample under investigation.

The lattice parameter of the FCC lattice is changed as function of the polar angle. You look for the polar angle where the experimental data (circles, not spirals) cross the spirals - hopefully all at once. In the image this is indicated by the blue line.

I would think that these days a computer is able to find lattice parameters without such visual aids. But the papers cited are surprisingly recent.

 

[mohammed.omar]

Thanks M Quack,

So as the polar angle is increased the distance from the different planes changes and that is why we get a Spiral? If tihs is the case then why do they all have minima at the vertical axis?

 

[mohammed.omar]

Or these spirals just come from spanning the lattice constant?

 

[M Quack]

spanning the lattice constant.

 

[mohammed.omar]

Thanks a lot M Quack