Refining cell

X-ray single crystal analysis Data collection. Siemens SMART IK CCD. Diffractometer. Data collection by SMART (Siemens, 1995) cell refinement SMART data reduction SAINT (Siemens, 1995) program(s) used to solve structure SHELXS97 (Sheldrick, 1997) program(s) used to refine structure SHELXL97 (Sheldrick, 1997) molecular graphics XP in SHELXTL (Sheldrick,... 

Pawley, G.S. Unit-cell refinement from powder diffraction scans. J.Appl. Cryst. 14 357-... 

Crystal color and size Reflections for cell refinement Data collection method diffractometer 9 range for data collection Index ranges... 

Figure 1. Results of full pattern fitting (after [53]) of the PXD pattern of an acid-treated phase A sample, based on an orthorhombic unit cell (refined lattice constants a = 7.940(3)A, b = 10.338(4)A, c = 11.591(5)A). The low angle peaks are somewhat asymmetric and are hence not fitted well by the assumed symmetric pseudo-Voigt peak shape.

Pawley, G.S., Unit Cell Refinement from Powder Diffraction Scans, Ibid. 14 357 (1981). 

The sample displacement parameter Ss calculated for each particular case from the displacement (s, in mm) obtained during unit cell refinement as Ss = -36000s/(7tR) or Ss = -144s/7T for the goniometer radius R = 250 mm. 

LAPOD Langford Unit Cell Refinement WWW. ccp 14. ac. uk/ccp/web-mirrors/lapod-langford/... 

XLAT Rupp Unit cell refinement ftp //www-structure.llnl.gov/... 

Nearly all unit cell refinement programs assume that a list of peak positions will be provided in some form. A personal favorite for quick unit cell refinement is the graphical Celref for Windows software of Jean Laugier and Bernard Bochu. Celref also understands space groups and includes a facility to automatically match HKL positions to observed peaks. For use of internal standard, the XLAT software of Bernhard Rupp, despite its age, is very effective. However,... 

Full profile fitting can be useful for various tasks including unit cell refinement involving peak overlap, space group assignment, extracting intensities prior to structure solution, and pre-structure refinement fitting of the powder pattern. The two main methods are listed below. 

Table 17.15 Available unit cell refinement software.

Unit cell refinement from powder diffraction data the use of regression diagnostics, T. J. B. 

PAW 81] PAWLEY G.S., Unit cell refinement from powder dif action scans , J. Appl. Cryst, vol. 14, p. 357-361,1981. 

There are problems with the unit cell refinement. 

Relax the fiexible degrees of freedom of the API and co-former by optimizing their intramolecular interactions, followed by cell refinement for a selection of likely structures. 

Let us finish this section by giving a practical example application of the superposition model to the compound Nd203. The space group is P3ml, the parameters of the hexagonal cell refined at 4K by Faucher et al. (1982) are a = 3.8277 A, c = 5.9908 A, with the positional parameters u = 0.2463, l> = 0.6469. The site symmetry of the rare earth is... 

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