6 XRD (X-ray Diffraction)

Question 1

what is XRD

Answer 1

-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

Question 2

what are the orbitals corresponding to emitted x rays

Answer 2

• 2p 3/2 = Ka1
• 2p 1/2= Ka2
• 3p= Kb

Question 3

what is bremsstrahlung radiation

Answer 3

-created non uniform background
- rad emitted when e decelerates as it encounters matter
-e slows as approach nuc, E releases as x-ray

Question 4

what do does K a or B and 1 or 2 stand for in Ka1

Answer 4

K = relaxes to 1s
a = from 2p
b= from 3p
1 or 2 = spin state

Question 5

what are the sources of unwanted rad and how to rid?

Answer 5

-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

Question 6

what is Braggs law?

Answer 6

diffracted rays either constructive or destructive
(constructive if 1 wavelength apart)

sin^2= lambda^2/(4a^2)(h^2+k^2+l^2)

Question 7

can you use peak intensity to do quantitative stiff?

Answer 7

no

can if have more info

Question 8

how to prepare sample for XRD

Answer 8

• ground to powder get small crystals with all possible orientation

get measure that is 2theta (important part)

Question 9

pros XRD

Answer 9

• fast, easy, cheap
• large library known samples

Question 10

what does amorphous look like?

Answer 10

large hump instead sharp peaks

Question 11

what is the Scherrer analysis

Answer 11

-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

Question 12

Systematic absences

Answer 12

-planes that reflect

• BCC: addition miller indices multiple of 2
• FCC: miller indices either all odd or even