After fitting the lattice parameters, view the difference between the observed and computed pattern. When peaks are well aligned, any remaining unfit intensity will appear largely symmetrical. When peaks are not well aligned, the difference has what I call a “derivative” shape, looking something like the first derivative of the peak. Compare Figs. 12.1 and 12.2. In 12.1 the difference curve shows that there is a bit of intensity in each peak that is not accounted for by the model, but the extra intensity is symmetrical around the peak position, while in 12.2 note how the difference curve starts positive below the peak but then goes negative above the peak. This latter curve is characteristic of what the difference curve will look like where the calculated peak is not aligned with the data. Small deviations in peak positions may remain, and require inclusion of the appropriate instrumental parameter to be fit, and there certainly may be significant differences between the observed and calculated peak intensities, but the the peaks largely should be aligned. If not, the question to consider will be, did not model not converge because it was not started close enough to the correct values for the lattice parameter or is this not the correct cell? In one structure that kept me stumped for weeks, the lower angle data for a fit of Faujasite gave me an excellent fit, but above some value, the peaks were poorly aligned. It turns out that the problem was that the symmetry of the material had dropped and the cell was now orthorhombic. I had not considered that because everyone who works with zeolites knows that Faujasite is cubic.
Assuming that the fit of the lattice parameters is good, add the appropriate sample parameters into the fit and again view the difference between the observed and computed pattern. There should be no indications that any peaks are misaligned.
