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Topic: the meaning of "d" in Bragg's law  (Read 1826 times)

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Offline blaisem

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the meaning of "d" in Bragg's law
« on: April 15, 2014, 09:00:40 AM »
Hi, there.

After reading the Wiki page on Bragg's Law, I noticed a small section under reciprocal space that states that the distance measured is not actually the distance between atoms in a lattice.

Given that, here's my understanding: When I solve for d in Bragg's law, the interplanar distance in a lattice would be equilavent to 1/d.  Is this correct?

Thank you!

Offline mjc123

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Re: the meaning of "d" in Bragg's law
« Reply #1 on: April 15, 2014, 11:19:13 AM »
No, the interplanar distance is d. It can't be 1/d because d has the dimensions of length in nλ=2dsinθ.
The distance d, as you say, is not the distance between atoms, but the separation between symmetry-equivalent planes of atoms, of which there are many sets.
In reciprocal space, the difference between the wavevectors of the incident and diffracted radiation is a reciprocal lattice vector whose magnitude is proportional to 1/d. So the smaller is d, the greater the angle of diffraction. The reciprocal lattice is an alternative representation of the information contained in the crystal structure, where each point represents a set of planes in the crystal, and therefore a spot in the XRD pattern.

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