Powder diffraction and refinement Flashcards
What is the purpose of whole-pattern fitting in structure refinement?
To analyze the overall diffraction profile without referencing a specific structural model, using approximate cell parameters.
What does the Rietveld refinement method minimize?
The function M = Σ wi(yi - yci)², where yi is observed intensity and yci is calculated intensity.
What are the essential outcomes of Rietveld refinement?
Refined cell parameters and line intensities from the diffraction data.
Define the difference between qualitative and quantitative phase analysis in X-ray diffraction.
Qualitative phase analysis identifies unknown phases, while quantitative phase analysis determines the weight fractions of phases present.
What is the significance of R-factors in structure refinement?
R-factors measure the agreement between observed data and refined models, indicating the quality of fit.
How does the mixing parameter η affect the pseudo-Voigt function?
It defines the shape of the peak, interpolating between Gaussian and Lorentzian forms.
What characterizes the Gaussian component in profile refinement?
It has a specific variation with scattering angle, influencing peak shape in diffraction patterns.
What are the advantages of utilizing neutron diffraction over X-ray diffraction?
Neutron diffraction provides larger contrasts between elements and better penetration into materials.
What are background intensities, and how are they estimated in neutron studies?
Background intensities are estimated using linear extrapolation or polynomial fitting to minimize noise effects in data.
What does the Williamson-Hall plot reveal in diffraction experiments?
It distinguishes and estimates crystallite size and lattice strain from diffraction data.
What is the basic equation for calculating intensity in constant wavelength diffraction?
Ih = 8Tr (Ior Is KR KG Va jh Lp (Fh²)
What does the structure factor Fh account for in diffraction patterns?
It encompasses contributions from atomic positions and their scattering amplitudes in the diffraction analysis.
How is refined resolution function related to structure refinement methods?
The refinement of resolution function accounts for instrumental characteristics during the fitting process.
What is meant by line overlap in powder diffraction patterns?
Line overlap occurs when peaks cannot be separated, complicating data refinement and interpretation.
Explain the significance of particle-size and microstrain broadening in diffraction.
They affect the shape and width of diffraction peaks, influencing interpretation of crystallite size and strain.
What is meant by the term ‘FWHM’ in diffraction line width measurement?
FWHM stands for Full Width at Half Maximum, quantifying the width of a diffraction peak at half its maximum height.
What components determine the line profile in powder diffraction?
The line profile is determined by the wavelength distribution, intrinsic instrument characteristics, and the diffraction profile of the sample.
How does sample broadening differ in neutron vs. X-ray diffraction?
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.
What is the purpose of the pseudo-Voigt function in diffraction analysis?
The pseudo-Voigt function is used to approximate the normalized Voigt function, simplifying the handling of peak shapes in fitting processes.
Describe the role of the Rietveld formula in profile fitting.
The Rietveld formula iterates to calculate integrated intensities based on overlapping Bragg reflections and the background, improving fitting accuracy.
What is the Williamson-Hall method used for in diffraction analysis?
The Williamson-Hall method helps to separately estimate crystallite size and lattice strain from the width of diffraction peaks.
How do neutrons interact differently with matter compared to X-rays?
Neutrons interact with atomic nuclei and provide high contrasts between elements, while X-rays primarily interact with electron clouds.
What is the significance of the structure factor Fh in diffraction patterns?
The structure factor Fh represents the scattering contributions from the atoms in the unit cell, crucial for calculating peak intensities.
Define ‘integrated intensity’ in the context of diffraction.
Integrated intensity is the total intensity of a diffraction peak obtained by calculating the area under the peak profile.
What does qualitative phase analysis using X-ray powder diffraction achieve?
Qualitative phase analysis matches experimental diffraction patterns to reference patterns to identify unknown phases in a sample.
