Diffraction

Question 1

What are the diffraction indices?

Answer 1

hkl specifying the set of equidistant planes.

Question 2

What is the wavelength of the often used CuKa radiation?

Answer 2

1.54056 Å

Question 3

What is Braggs law?

Answer 3

λ = 2 d_hkl sin θ

λ = wavelength of radiation used
d_hkl = interplanar distance between two hkl-planes
θ = angle of incidence

Question 4

Where is the scattering factor f strongest?

Answer 4

For glancing angle θ = 0. It decreases exponentially with increasintg sin θ / λ

Question 5

How does a basis change the reflection conditions?

Answer 5

There will be no new reflections. A basis consisting of several atoms (e.g. 3) can be interpreted as three primitive bases oriented the same way, which will give the same reflections. However, there will be a phase change between the reflections of the different atoms in the basis, so that we might have increased intensities for some peaks (where all waves are in phase) or a destructive interference so that certain reflections are diminished (extinguished if the atoms in the basis are identical).

Question 6

What are systematic absences?

Answer 6

Combinations of hkl where the structure factor is 0.

Question 7

How are systematic absences listed in the International Tables of Crystallography?

Answer 7

Instead of listing the absences, they list the conditions that must occur for diffraction to happen, so called diffraction conditions.

Question 8

What is the multiplicity factor?

Answer 8

When several sets of equidistant planes are symmetrically equivalent (with different orientation). In NaCl, 111 planes are found in 8 different directions. The multiplicity factor for these planes is 8.

Question 9

What is the Lorentz-polarization factor?

Answer 9

A factor that concerns the fact that diffraction in general polarizes the reflected beam. The energy loss depends on the diffraction angle.