6 XRD (X-ray Diffraction) Flashcards
what is XRD
-x-ray diffraction (powder diffraction)
-x-ray diffracted from crystal
- high E e (usually Mo or Cu), e ejected, e relaxes, x-ray emitted
- get diffractogram
what are the orbitals corresponding to emitted x rays
- 2p 3/2 = Ka1
- 2p 1/2= Ka2
- 3p= Kb
what is bremsstrahlung radiation
-created non uniform background
- rad emitted when e decelerates as it encounters matter
-e slows as approach nuc, E releases as x-ray
what do does K a or B and 1 or 2 stand for in Ka1
K = relaxes to 1s
a = from 2p
b= from 3p
1 or 2 = spin state
what are the sources of unwanted rad and how to rid?
-Kb and bremsstrahlung rad
- use filters 1 or 2 atomic numbers below
- or crystal oriented at specific angle
Cu for Kb and Ni for Brem
- Ka1 and Ka2 appear monochromatic, can’t get rid of it
what is Braggs law?
diffracted rays either constructive or destructive
(constructive if 1 wavelength apart)
sin^2= lambda^2/(4a^2)(h^2+k^2+l^2)
can you use peak intensity to do quantitative stiff?
no
can if have more info
how to prepare sample for XRD
- ground to powder get small crystals with all possible orientation
get measure that is 2theta (important part)
pros XRD
- fast, easy, cheap
- large library known samples
what does amorphous look like?
large hump instead sharp peaks
what is the Scherrer analysis
-estimate size nano crystal (less100nm and spherical)
- particles smaller fewer planes produce constative interference, get broader peaks
above 200nm other broadening effects and can’t analyze
- complementary tech, only sees small part
Systematic absences
-planes that reflect
- BCC: addition miller indices multiple of 2
- FCC: miller indices either all odd or even