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

(This Webpage Page in No Frames Mode)

CCP14

Tutorials and Examples

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

Empirical 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] | [Fundamental Parameters Fitting]

[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 Empirical peak profiling and XFIT to obtain peak information for auto-indexing and structure factor extraction for Patterson based structure solution. Peak 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))


Download Data

Click here to download a zip file of the Liti data

The 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.

Fitting lower angle peaks


Fitting higher angle peaks

This results in a list of peaks with ESDs. To generate ESS, re-fit the range of peaks of interest, then from the menu do Fit, Errors. Split Pearson peaks can have large ESDs on parameters such as peak area, FWHM due to the flexibility and nuances of this type of peak profile function (e.g., unconstrained peak fitting can give extremely long tails). Go into File Details, Peaks, (Split Pearson and Psuedo Voight) to get the results). Note the ESDs on the PVII (Split Pearson) due to unconstrained peak fitting using this function on small peaks.

  •      File       Peak     Area      ESD        Th2     ESD 
                    Type
    tl-15.dat       PVII   99.9593       80     12.5525 0.0282
                    PVII   588.1507      20     18.1033 0.0068 
                    PVII  1042.0878     118     21.6934 0.0066  
                    PVII  1586.0946      27     22.4978 0.0040 
                    PVII   151.4866    2347     25.3291 0.0279 
                    PV     930.6631      13     30.6681 0.0017
                    PV     548.2765      10     31.8372 0.0020
                    PV     863.7153      26     35.9310 0.0033
                    PV     129.6226      34     36.2263 0.0102
                    PV     158.8826      16     36.6456 0.0078
                    PV     65.9024        7     38.3090 0.0072
                    PV     20.9200       12     39.3811 0.0168
                    PV     46.7854       32     40.1551 0.0097
                    PV     122.7227      73     40.4303 0.2552
                    PV     542.1722      83     40.8613 0.0026
                    PV     123.0962      12     42.0209 0.0054
                    PV     13.7453        4     43.3407 0.0187
                    PV     92.0356       22     44.1615 0.0200
                    PV     84.1511       23     44.4226 0.0489
                    PV     49.1961        5     45.9303 0.0102
                    PV     90.4678        6     47.8481 0.0072 
                    PV     144.5448       7     48.8640 0.0055
                    PV     294.4215      10     51.7317 0.0044
                    PV     763.1232      72     52.8917 0.0048 
                    PV     119.8731      71     53.2360 0.1763
                    PV     305.5976       9     55.2187 0.0037 
                    PV     333.8850      26     57.9923 0.0064 
                    PV     79.8515       43     58.3830 0.0529
                    PV     259.2144      15     59.2134 0.0051 
                    PV     91.2168        9     61.6432 0.0104
                    PV     164.7747      10     62.9206 0.0064
                    PV     87.0643       12     63.7716 0.0119
                    PV     147.6010      25     65.3505 0.0091
                    PV     249.0785      36     65.8221 0.0425
                    PV     365.9529      12     67.7026 0.0048
                    PV     34.5275       14     68.5652 0.0162 
    


    [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