[CCP14 Home: (Frames | No Frames)]
CCP14 Mirrors: [UK] | [CA] | [US] | [AU]

(This Webpage Page in No Frames Mode)


Tutorials and Examples

Peak Fitting using Xfit-Koalariet (Coelho and Cheary) for Win95/NT

Fundamental Parameters Peak Profiling on lab Powder XRD data for Ab-initio Indexing and Structure Solution

The CCP14 Homepage is at http://www.ccp14.ac.uk

[Tutorials page] | [XFIT Tutorial Index | [Empirical Fitting Using XFIT]

[The reference to use for XFIT or FOURYA in any resulting publications is: Cheary, R. W. & Coelho, A. A. (1996). Programs XFIT and FOURYA, deposited in CCP14 Powder Diffraction Library, Engineering and Physical Sciences Research Council, Daresbury Laboratory, Warrington, England. (http://www.ccp14.ac.uk/tutorial/xfit-95/xfit.htm)]

The Problem

Following is a real world example of using Fundamental Parameters peak profiling and XFIT to obtain peak information for auto-indexing and structure factor extraction for (in this case) Patterson based structure solution. Peaks are defined visually. If there is a significant misfit, this could imply an extra peak is present but care needs to be taken. For difficult problems, indexing in done iteratively with the peak fitting to check for possible impurity phases. The data is of a new Lithium Titanate (I.E. Grey, L. M. D. Cranswick, C. Li, L. A. Bursill, and J. L. Peng, "New Phases Formed in the Li-Ti-O System under Reducing Conditions", Journal of Solid State Chemistry, 138, 74-86 (1998)) The advantage of fundamental parameters over Empirical peak fitting is more confidence in fitting real peaks due to being inherantly able to model real peak asymmetry due to the diffractometer geometry; and more of a handle in terms of trying to understand what any misfits could be due to. Also, as you are trying to refine on physically meaningful parameters, parameters such as crystallite size and crystallite strain can be used as to describe the width/shape of all the peaks resulting in only 2 parameters per peak instead of 4 or greater with empirical peak modelling methods.

Download Data

Click here to download a zip file of the Liti data

The powder X-ray Diffraction data is a fast 20 minute continuous scan from 5 to 70 degrees run on a Philips Diffractometer. The reason this data was used was that the sample degraded somewhat by the time a slow Rietveld quality step scan had been run. Rather than twiddle thumbs for the night, it was decided to use the continous scan data. It eventuated that not only could this quick continuous scan data index the phase and assign a space group from the absences; but fitted intensities allowed the structure to be solved with Shelx using Pattersons down to the Oxygens. A new preparation, Rietveld quality XRD data, combined with difference map Fourier contour maps inside the CSRIET software, allowed refinement down to the Lithiums. Neutron data was used to confirm the position and reliability of the Lithium positions.

While the Philips APD 3.6 for DOS software was used for peak profiling on the original structure solution, this goes through the methodology using freely available software.

The scan was collected on a Philips 1710 system with 1050 goniometer (173mm goniometer radius) with Cu LFF X-ray tube at 40mA and 40kV, incident beam Sollers slit with 5.1 degree acceptance angle, no diffracted beam Sollers slit (equivalent of 9.1 degree acceptance angle) 1 degree fixed divergence slit, 0.45mm receiving slit, 1 degree scatter slit, diffracted beam curved graphite monochromator and proportional counter. Continous 20 minute scan from 5 to 70 degrees at 0.025 steps. Slit lengths are 12mm (wide optics) and sample length is ~25mm.

One thing to keep in mind is that some of the misfit could be slight degradation of the sample. Which can make things awkward. Please note that if there is a "new" effect that is not included in the fundamental parameters

Note: Remember that when you add a peak using the fundamental parameters method and XFIT, you have to go back into "File Details, Peaks, Fundamental Parameters Peaks"; change the strain from the default of 0, to match the peaks you are linking this to (or 0.01 if freely refined); change the crystallite size to the peaks you are linking this to; "Use" the peak, then enable the refinement flags for these newly inserted peaks. Tedious compared to the Empirical peak fitting method but such is life.

Setting up the Cu X-ray Emission Profile, Diffractometer and Background Parameters

Selecting and Fitting the Peaks

Examining the Peak Fit

Obtaining Results

     File        Peak     Area    ESD      Th2      ESD
tl-15.dat       FP      73.7972  7.04     12.5833  0.0152
                FP     525.1695 11.33     18.1219  0.0031 
                FP     808.8842 13.64     21.7033  0.0024
                FP    1617.7630 18.58     22.5138  0.0016
                FP      29.2490  4.03     25.2852  0.0212
                FP     842.7766 13.50     30.6925  0.0022
                FP     530.1695 10.90     31.8536  0.0027 
                FP     755.3265 15.95     35.9364  0.0031 
                FP     194.3486 11.40     36.2205  0.0091 
                FP     135.4252  6.50     36.6579  0.0072
                FP      61.5260  4.51     38.3112  0.0105
                FP      23.0393  3.40     39.4055  0.0230
                FP     115.8754  5.67     40.1829  0.0076
                FP     557.0820 11.02     40.8568  0.0027
                FP     116.2905  5.50     42.0205  0.0069
                FP      12.0952  2.91     43.3329  0.0378
                FP     106.6417  6.94     44.1745  0.0104
                FP      50.8949  6.00     44.4607  0.0192
                FP      43.0744  3.88     45.9254  0.0139   
                FP      81.4082  4.82     47.8408  0.0091 
                FP     126.3955  5.72     48.8521  0.0068
                FP     259.5472  8.78     51.6883  0.0057 
                FP      46.9653  5.74     52.0403  0.0238 
                FP     715.0383 13.28     52.8705  0.0030 
                FP      82.8354  6.46     53.2775  0.0153
                FP     278.5762  8.10     55.1945  0.0045
                FP     333.2409  9.23     57.9765  0.0045
                FP      56.8873  5.20     58.4382  0.0181
                FP     222.0052  7.47     59.1860  0.0054
                FP      69.8183  4.94     61.6024  0.0125
                FP     144.1896  6.40     62.8888  0.0074
                FP      74.4343  5.09     63.7226  0.0128
                FP     212.7719  7.90     65.3322  0.0068
                FP      90.6220  6.80     65.8148  0.0177
                FP      33.6637  5.76     66.1785  0.0374
                FP     301.1027  8.75     67.6631  0.0049    
                FP      47.2308  5.10     68.5051  0.0179

[Tutorials page] | [XFIT Tutorial Index]

[CCP14 Home: (Frames | No Frames)]
CCP14 Mirrors: [UK] | [CA] | [US] | [AU]

(This Webpage Page in No Frames Mode)

If you have any queries or comments, please feel free to contact the CCP14