Sample Preparation, Data Collection and Phase-ID using Powder XRD. Pamela Whitfield Canadian Powder Diffraction Workshop. Horses for Courses…. Data quality required depends on what you want to do with it Phase-ID has less stringent requirements on both sample prep and data collection
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Canadian Powder Diffraction Workshop
Note: convention is that –ve sample displacement = sample too high
Not an issue for parallel beam systems
Crystallite Powder XRD
Crystallites / 20mm3
5.97 × 105
3.82 × 107
3.82 × 1010
No. of diffracting crystallites
Comparison of the particle statistics for samples with different crystallite sizes
Reproducibility of the intensity of the quartz (113) reflection with different crystallite sizes
McCrone mill = good
Mortar and pestle = bad
Overlay of 3 repeat patterns from un-micronized cement
Overlay of 3 repeat patterns from micronized cement
m = linear absorption coefficient (LAC)
Alternative holders such as zero background silicon or quartz usually top-loading as well
Gem Dugout a commonly used source for zero background plates (www.thegemdugout.com)
Effect on the QPA XRD results. Kinetics from reflection data nonsensical.
N.B. Texture Index of 1 = perfect powder.
(ENeqV1_0.xls very handy for working out correct divergence)
0.01º step, 1s count
Rwp = 15.2%
0.02º step, 2s count
Rwp = 12.0%
2 different datasets from quartz stone
– both experiments took 25 seconds
Smaller Rwp corresponds to a better fit.
→ greater uncertainty in structural parameters and quantitative phase analysis
Which line would you choose?
1300 Powder XRD
CuKa - Li1.15Mn1.85O3.9F0.1
2-Theta - ScaleFluorescence
there is a real
CoKa - LiMn1.5Ni0.5O4
Which dataset do you prefer?
P/B = 13.4
Rescaled to normalize background
P/B = 4.5
Sacrifice intensity to improve P/B ratio
P/B = 4.2
P/B still along way off 50. Change radiation or instrument.
Comparison of data from aspirin using lab top-loading and capillary compared to synchrotron data.
P/B = 4.5
P/B = 9.4
P/B = 84
P/B = 87
(P/B = 54 without air-scatter sink to reach angles >100)
A primary monochromator would get rid of this high angle tail
Data from the mineral stichtite
Raw VCT capillary data for stichtite
Data reformatted into ASCII format xye file
Remember if subtracting background (e.g. capillary blank) that the error is original intensity!
Fixed count time
Variable count time (normalized)
Example from presentation by Lachlan Cranswick
Jadarite structure with thermal ellipsoids
Search-match in EVA on a sample of zircon
a = 5.456, c = 13.143Å
a = 5.449, c = 13.104Å
Questions? isn’t the best one for the job!