Ab initio structure solution

EDXRD is a very powerful technique, although limitations include the requirement for synchrotron radiation. This Hmits the number of experiments that can be performed, due to the high cost and low availabihty of synchrotron beam time. Because of the large voliune of the reaction vessel and the geometry of the instrument, the peak resolution of the energy dispersive data is also rather poor (AE/E). This means that although it is possible to accurately monitor the course of a reaction, using the data for ab initio structure solution or structure refinement is precluded. 

Cone-angle dependence of Ab-initio structure solutions using precession electron diffraction. AIP Conf Proa, 999, 53-65. 

Chowdhury K., Bhattacharya, S. and Mukherjee, M. (2005). Ab initio structure solution of nucleic acids and proteins by direct methods reciprocal-space and real-space approach. /. Appl. Cryst. 38,217-222. 

Frazao, C., Sieker, L., Sheldrick, G. M., Famzin, V.,LeGall,J. and Carrondo, M. A. (1999). Ab initio structure solution of a dimeric cytochrome c3 from Desulfovibrio gigas containing disulfide bridges. /. Biol. Inorg. Chem. 4, 162-165. 

Putz, H., Schon, J. C. and Jansen, M. (1999). Combined method for ab initio structure solution from powder diffraction data. J. Appl. Crystallogr, 32, 864-70. [Ill] Ramamurthy, V. and Venkatesan, K. (1987). Photochemical reactions of organic crystals. Chem. Rev., 87,433-81. [236]... 

By solving crystal structures of different classes of materials," we will illustrate only a few of the possible approaches to the ab initio structure solution from powder diffraction data. Whenever possible the structure factors obtained from full pattern decompositions should be used until the coordinates of all atoms are established. In some cases it may be necessary to re-determine individual structure factors based on the nearly completed structural model, especially when locations of lightly scattering atoms are of concern after all strongly scattering species have been correctly positioned in the unit cell. This re-determination may be routinely performed during Rietveld refinement and will be briefly discussed in Chapter 7. 

The fact that a total of 16 ( ) space groups are possible for this material could make it a complicated choice for the ab initio structure solution. However, its unit cell is quite small. Furthermore, LaNi4.g5Sno.15 is an intermetallic compound and therefore, the highest symmetry space group (P6/mmm) is quite probable. 

It is important, however, to remember that the Rietveld method requires a model of a crystal structure and by itself offers no clue on how to create such a model from first principles. Thus, the Rietveld technique is nothing else than a powerful refinement and optimization tool, which may also be used to establish structural details (sometimes subtle) that were missed during a partial or complete ab initio structure solution process, i.e. as in the twelve examples described in Chapter 6. 

Putz H, Schon JC, Jansen M (1999) Combined method for ab initio structure solution from powder diffraction data. J Appl Crystallogr 32 864-870... 

Endeavour Combined Method for Ab initio Structure Solution from Powder Diffraction Data, H. Putz, J. C. Schoen, M. Jansen, J. Appl. Crystallogr., 1999, 32, 864 870 Direct space and energy minimization... 

A structural model obtained either from a database (such as ICSD, CCDC, etc.) or from the ab initio structure solution method is refined with the observed powder pattern by a nonlinear least squares method to obtain accurate values of various parameters involved. The residual, Sy, is minimized ... 

Perhaps the most difficult task of powder X-ray diffraction studies is ab initio structure solutions. Unfavourable scattering characteristics, disorganized water molecules, impurities, inhomogeneities, twinning, stacking disorders, and pseudosymmetries further complicate analyses. Furthermore, as heteroatoms can occupy... 

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