X-ray Powder Diffraction

What do crystalline material produce in XRPD

produce "Bragg peaks"

What do crystalline material produce in XRPD

amorphous materials produce a "halo"

What can XRPD do?

Can easily distinguish between solid forms including polymorphs

Can quantify crystallinity (how much)

What are x-rays?

•Waves (radiomagnetic) with wavelength around 0.1 nm

•X-ray labs often use Cu (copper) as a source of X-rays

o0.154 nm wavelength

•C-C bond length about 0.154 nm

Theory-diffraction

Object > wavelength = shadowing (your hand and light)

Object < wavelength = not visible (atoms and light)

Object similar size to wavelength = diffraction

What is bragg’s law

diffraction happens at angles where

n=1, lambda is wavelength

d is spacing between planes of atoms

theta is (half) the scattering angle

Summary

•Bragg peaks -> crystalline

•Broad "halo" -> amorphous

•Easily distinguishes between different solid forms, including ones with same composition (polymorphs)

•Diffraction – object has similar size to wavelength being used

•Bragg equation - no derivation provided but lots available online

oRelies on constructive vs destructive interference of X-ray waves

XRPD – gold standard for materials analysis of solids

questions

•Why are XRPD patterns unique for each crystalline substance?

•What happens at a diffraction angle where the Bragg equation is not satisfied?

•How does XRPD help in identifying the polymorphic form of a drug?

•If a sample contains 80 % crystalline material and 20 % amorphous, what would the pattern look like?

•Why pattern, why not spectrum?

•How might an increase in temperature affect a XRPD pattern?