DICVOL04

Trial and error method for the automatic indexing of Powder diffraction patterns. Variation of parameters by succesive dichotomy.

DICVOL04 use as input data all available diffraction lines but only the first n (e.g. 20) lines will be used for the search of solutions. These solutions are then used automatically for reviewing all input data. The impurity tolerance 'imp' is related only to the n lines used for searching the solution (of course, additional unindexed lines can be found among the extra input lines in the reviewing process). Use with care the impurity tolerance: spurious lines increases the risk to miss the correct solution.

DICVOL04 offers the possibility to analyse your input data for the presence of a zero-point error. it can suggest more than one or two solutions (due to uncertainty in searching harmonic diffraction lines). This option is to be used prior the indexing process. please, as far as possible avoid the use of this facility by ensuring the quality of your collected data.

The following crystalline systems can be examined: Cubic, tetragonal, hexagonal, orthorhombic, monoclinic and triclinic.

 

Recommendations

• Read carefully the user guide of DICVOL04. Please, spend time to ensure the quality of your observed data with accurate data the success rate of DICVOL04 is very high. Peak positions should be extrated with a profile fitting software. An interactive program should be preferred, since automatic extractions can miss lines (low intensity, shoulder, ...)
• With bad data, the chance to obtain the correct solution is very small and the calculation can be time-consumming
• With modern x-ray powder diffractometers (the use of monochromatic radiation is strongly recommended) absolute errors on peak positions lower than 0.02 degrees 2-theta can be routinely obtained. For indexing purposes, errors should not(ideally) exceed 0.03 deg. 2-theta.
  [In exceptional cases, a few lines with greater individual estimated error can be introduced in input data. In this case, use the parameter eps= 1. in card 5, and enter individual errors on each line
• With high resolution powder diffraction data (conventional or, particularly, synchrotron x-ray sources), the absolute error is usually less than 0.02 (or even 0.01) deg. 2-theta; consequently, eps=0.02 (or even eps=0.01) is recommended; the convergence of the dichotomy procedure will be improved. however, be sure that this condition is true for all lines used as input data
• Note that the program dicvol04 is executable from 7 lines - 8 lines if zero-shift is refined - (though it is not recommendable since L-S refinement instabilities can be expected)
• In DICVOL04, solutions are searched for the smallest cell volumes. As soon as a solution is found, only solutions with smallest volumes are retained.
  The search is exhaustive within the input limits.
  If (for some reasons!) you are not satisfied by the solution, you can run again the program with an input lower volume limit slightly greater than that of the found solution (the exhaustive search is then extended to a higher volume)
• Note that the search is exhaustive within the limit on input data. In particular, the search is constrained by the higher and smaller bounds on parameters, volumes, selected fom and absolute errors. Please act on these parameters when using DICVOL04
• Check on validity of an indexing result
• Possible space groups: look at the absent reflections in the output list of the reviewing of the complete input data

 

For further information contact

D. LOUER
Laboratoire de Chimie du Solide et Inorganique Moleculaire
Groupe de Cristallographie des Poudres et Reactivite des Solides
Universite de Rennes I
Campus de Beaulieu
35042 RENNES CEDEX - FRANCE
E-MAIL Daniel.Louer@free.fr


Or

A. BOULTIF
Laboratoire de Cristallographie
Departement de Physique
Faculte des Sciences
Universite Mentouri - Constantine
25000 CONSTANTINE - ALGERIE
E-MAIL ABoultif@caramail.com