| Summary: | The quality of X-ray powder diffraction data and the number and type of
refinable parameters has been examined with respect to their effect on
quantitative phase analysis (QPA) by the Rietveld method using data collected
from two samples from the QPA round robin [Madsen et al. J. Appl. Cryst.
(2001), 34, 409-26]. For specimens where the diffracted intensity is split
between all phases approximately equally accurate results could be obtained
with a maximum observed intensity in the range of 1000 - 200000 counts. The
best refinement model was one that did not refine atomic displacement
parameters, but did allow other parameters to refine. For specimens where there
existed minor or trace phases, this intensity range changed to 5000 - 1000000
counts. Contrastingly, here, the refinement model with the most accurate
results was one that refined a minimum of parameters, especially for the
minor/trace phases. Given that all phases had quite narrow peaks, step sizes
for both types of specimen could range between 0.01 - 0.04 deg2th, and still
yield acceptable results. Data should be collected over a 2Th range that
captures the lowest angle peak, and continues at least until (i) there is a
constant increase in cumulative intensity with angle, (ii) a point where peaks
no longer appear, or (iii) the upper 2Th limit of the goniometer. The wide
range of these values show that QPA by the Rietveld method is quite robust with
regards to data quality. As these are ideal specimens, these values indicate a
best-case scenario for the collection of diffraction data for QPA by the
Rietveld method, but does show that the analysis can be quite forgiving of
lower quality data.
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