This example uses Li_0.92Ti_2.88O₆ as published in (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)).

It should be noted that this structure was originally solved by manual interpretation of a 3D Patterson map generated by Shelx76.

Also refer to Armel LeBail's Tutorial on solving structures from powder data

• Original 1 at http://www.cristal.org/iniref/tutorial/indexa.html
• Original 2 at http://fluo.univ-lemans.fr:8001/iniref/tutorial/indexa.html
• CCP14 Web Mirror

Starting Information

• Wavelength of radiation used.
  • 1.54056 Angstrom (Cu K-alpha)
• Space group
  • c 2/m (published in A 1 1 2/m to make it more comparable to related structures)
  • (It should be noted that if you have not determined the space group, WinGX can try using ABSEN authored by P McArdle to try and determine the spacegroup). In this particular case, the space group was determined manually. In the case of overlap problems, Le Bail fitting on various monoclinic spacegroups and using the overall profile fit as a guide is another possible method.

    The UNIT CELL was determined using ITO, though a tutorial on using the three main powder indexing programs using the CRYS2RUN system is available using this particular data.

• Chemical Composition/Formula:
  • Li 0.92, Ti 2.88, O6 (round these numbers for EXPO)
• Number of Formula Units per Unit Cell
  • Z = 2
• Thus the Unit Cell contents are:
  • Li 2 O 12 Ti 6
• Unit Cell Dimensions:
  • a=14.0956(0.002) b=2.9524(0.002) c=4.8917(0.001) beta=92.1716(0.05)

From either an Icon in MS-Windows (or typing expo on UNIX command line), run expo.

• This brings up the following screen

• 

• Click on OK and you are then prompted to open up a EXPO control file.

• 

• The file we are going to look at here is described above, liti. Following is the information inside the file that you would be expect to edit manually and is described in the EXPO manual [Home Site] | [CCP14 Mirror].

  %struct liti
  %job Lithium Titanate - Ian Grey, et al.
  %init
  %data
  range  10.000   150.000  0.025 
  pattern liti.pow
  cont  Ti    6  O    12  Li    2 
  wave 1.54056
  cell     14.0956   2.9524   4.8917  90.000  92.1716  90.000
  space c 2/m
  alpha 
  %extraction
  %continue
  

• Click on the Browse to view what input files are available and click on the liti.exp file.

• 

• Click on OK to confirm your selection.

• 

• In this particular dataset, there is a trace impurity, but we are ignoring it.
• Then click OK again to let EXPO go through its stuff (like Sir97 this is the fun to watch bit). At various points, the program stops to give you the choice of changing the defaults. In this example, all you have to do is keep pressing the CONTINUE button.

  Even though EXPO is prompting for a Range, pressing CONTINUE EXPO will apply an Alpha 2 correction

  At this point, you can redefine the range of the data on which the structure factor extraction is performed. In this case, we may as well just go up to the full range but in some cases, this is a variable to play with as above a certain angle, the overlap can be so excessive as to be useless.

• 

• Click continue and EXPO will select what it considers to be the best intervals for the analysis. You can over-ride this if you which. For this example, we don't wish to over-ride anything, so just press CONTINUE

• 

• EXPO determines the background. You can zoom and inspect this, but we will just accept this and CONTINUE.

• 

• EXPO tries to determine the best peak profile to use, the goes about extracting intensities in the "regions" it selected - in this case "Region 2".

• 

• Now it is continuing onto "Region 3".

• 

• Now EXPO does an extraction cycle on the full pattern.

• 

• EXPO is now finished the extracting of intensities and now we can proceed for the direct methods by pressing CONTINUE. In this case, if EXPO queries if you want to use pseudo-translational effects, just say no - but normally you would be checking these options out.

• 

• EXPO now is Phasing.

• 

• EXPO is now attempting to place Atoms.

• 

• EXPO now is assigning elements to the atoms (in this case, Pink is Titanium, Light Blue is Oxygen).

• 

• EXPO now presents a structure to us.

• 

• As with Sir97, we have the option to add Atom labels by using the PlotStyle Menu Option.

• 

• On quitting EXPO produces a *.xyz file with the co-ordinates that can be input into any other refinement/structure checking program for seeing if this is a reasonable solution. Bond angles and lengths, as well as Fourier maps can be examined from within the EXPO menu system.

• Lithium Titanate - Ian Grey, et al.                                             
   number of atoms =     5 for set no.     1
  
     type          x       y       z       Int.     occ.       B
  
       ti         .347    .000    .983     1488.    .500      2.364
       ti         .500   -.500    .500     1435.    .250      1.739
       o          .430    .000    .686      455.    .500      2.269
       o          .592   -.500    .804      447.    .500      1.461
       o          .272    .500    .800      428.    .500      1.672