Powder diffraction and refinement

Question 1

What is the purpose of whole-pattern fitting in structure refinement?

Answer 1

To analyze the overall diffraction profile without referencing a specific structural model, using approximate cell parameters.

Question 2

What does the Rietveld refinement method minimize?

Answer 2

The function M = Σ wi(yi - yci)², where yi is observed intensity and yci is calculated intensity.

Question 3

What are the essential outcomes of Rietveld refinement?

Answer 3

Refined cell parameters and line intensities from the diffraction data.

Question 4

Define the difference between qualitative and quantitative phase analysis in X-ray diffraction.

Answer 4

Qualitative phase analysis identifies unknown phases, while quantitative phase analysis determines the weight fractions of phases present.

Question 5

What is the significance of R-factors in structure refinement?

Answer 5

R-factors measure the agreement between observed data and refined models, indicating the quality of fit.

Question 6

How does the mixing parameter η affect the pseudo-Voigt function?

Answer 6

It defines the shape of the peak, interpolating between Gaussian and Lorentzian forms.

Question 7

What characterizes the Gaussian component in profile refinement?

Answer 7

It has a specific variation with scattering angle, influencing peak shape in diffraction patterns.

Question 8

What are the advantages of utilizing neutron diffraction over X-ray diffraction?

Answer 8

Neutron diffraction provides larger contrasts between elements and better penetration into materials.

Question 9

What are background intensities, and how are they estimated in neutron studies?

Answer 9

Background intensities are estimated using linear extrapolation or polynomial fitting to minimize noise effects in data.

Question 10

What does the Williamson-Hall plot reveal in diffraction experiments?

Answer 10

It distinguishes and estimates crystallite size and lattice strain from diffraction data.

Question 11

What is the basic equation for calculating intensity in constant wavelength diffraction?

Answer 11

Ih = 8Tr (Ior Is KR KG Va jh Lp (Fh²)

Question 12

What does the structure factor Fh account for in diffraction patterns?

Answer 12

It encompasses contributions from atomic positions and their scattering amplitudes in the diffraction analysis.

Question 13

How is refined resolution function related to structure refinement methods?

Answer 13

The refinement of resolution function accounts for instrumental characteristics during the fitting process.

Question 14

What is meant by line overlap in powder diffraction patterns?

Answer 14

Line overlap occurs when peaks cannot be separated, complicating data refinement and interpretation.

Question 15

Explain the significance of particle-size and microstrain broadening in diffraction.

Answer 15

They affect the shape and width of diffraction peaks, influencing interpretation of crystallite size and strain.

Question 16

What is meant by the term ‘FWHM’ in diffraction line width measurement?

Answer 16

FWHM stands for Full Width at Half Maximum, quantifying the width of a diffraction peak at half its maximum height.

Question 17

What components determine the line profile in powder diffraction?

Answer 17

The line profile is determined by the wavelength distribution, intrinsic instrument characteristics, and the diffraction profile of the sample.

Question 18

How does sample broadening differ in neutron vs. X-ray diffraction?

Answer 18

In neutron diffraction, sample broadening is often negligible due to instrumental resolution, while in X-ray diffraction, it is more significant due to better resolution.

Question 19

What is the purpose of the pseudo-Voigt function in diffraction analysis?

Answer 19

The pseudo-Voigt function is used to approximate the normalized Voigt function, simplifying the handling of peak shapes in fitting processes.

Question 20

Describe the role of the Rietveld formula in profile fitting.

Answer 20

The Rietveld formula iterates to calculate integrated intensities based on overlapping Bragg reflections and the background, improving fitting accuracy.

Question 21

What is the Williamson-Hall method used for in diffraction analysis?

Answer 21

The Williamson-Hall method helps to separately estimate crystallite size and lattice strain from the width of diffraction peaks.

Question 22

How do neutrons interact differently with matter compared to X-rays?

Answer 22

Neutrons interact with atomic nuclei and provide high contrasts between elements, while X-rays primarily interact with electron clouds.

Question 23

What is the significance of the structure factor Fh in diffraction patterns?

Answer 23

The structure factor Fh represents the scattering contributions from the atoms in the unit cell, crucial for calculating peak intensities.

Question 24

Define ‘integrated intensity’ in the context of diffraction.

Answer 24

Integrated intensity is the total intensity of a diffraction peak obtained by calculating the area under the peak profile.

Question 25

What does qualitative phase analysis using X-ray powder diffraction achieve?

Answer 25

Qualitative phase analysis matches experimental diffraction patterns to reference patterns to identify unknown phases in a sample.