Recommendations
- Read carefully references 1 and 6 (and 9), as well as this user's guide.
- Please, spend time to ensure the quality of your collected data (see ref. 5). With accurate data, the success rate of Dicvol06 is very high. Peak positions should be extracted 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 small and the calculation can be time-consuming.
- With modern x-ray powder diffractometers (the use of monochromatic radiation is recommended), absolute errors on peak positions lower than 0.02 degrees 2θ can be routinely obtained. For indexing purposes, errors should not (ideally) exceed 0.03º in 2θ. [In exceptional cases, a few lines with greater individual estimated error can be introduced in the input data. In this case, use the parameter EPS= 1, and enter individual errors after D(i) for each line].
- With high resolution powder diffraction data (conventional or, particularly, synchrotron x-ray sources), the absolute error is usually less than 0.02º (or even 0.01º with ultra-high resolution) in 2θ; 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. (Remember that all mathematical solutions within the input limits and error bounds are found, the greater they are the greater is the number of mathematical solutions).
- N_IMP can be used in case of expected spurious lines (i.e. impurity lines, as well as observed lines out of the input error). N_IMP acts at all successive levels of the dichotomy algorithm. As soon as an indexing solution is retained, a least-squares refinement of lattice parameters is carried out. For this refinement a larger error on observed lines is considered. Then, a line rejected at the last dichotomy level can, by chance, be accepted with the refined lattice parameters. [Example of possible case: N_IMP = 0 no solution; N_IMP = 1 one solution; however, after LS refinement of the lattice parameters all N lines used for searching solutions are indexed with the refined parameters.]
- Note that the program Dicvol06 is executable from 7 lines- 8 lines if the 'zero-shift' is refined - (though it is not recommendable since LS refinement unstabilities can be expected).
- Long and short axis cases (dominant zone cases): if such cases are expected, the number N of lines used for searching the solution should, generally, be greater than 20.
- The minimum value for a linear lattice parameter has been fixed to 2.5 angstroms.
- Reliability of indexing solutions: read paragraph 8 of ref. 5 and refs 7 and 8.
- Note that with the option Dicvol04 (option =0), as soon as a solution is found, only solutions with smallest volumes will be subsequently retained. 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 limits on the input data. In particular, the search is constrained by the higher and smaller bounds on parameters, volumes, selected FOM and absolute errors on peak positions. Please act on these parameters when using Dicvol06.
- A lattice metric singularity occurs when unit cells defining two lattices have an identical set of calculated d-spacings. This can be observed with high symmetry lattices (read ref. 5, sect. 4.2, and refs therein). simple relations exist between the parameters of the two cells, as well as particular cell-volume ratios. a typical case is: an hexagonal cell [a, c, volume v] can be indexed with an orthorhombic cell [parameters: a/2, a sqrt(3), c, volume v/2]. Due to the strategy used in Dicvol, based on an analysis through decreasing symmetry, all cells should be, in principle, displayed in the output file (except if a solution is rejected by the input maximum volume).
- Check on validity of an indexing result: please read ref. 5 (sect. 8)
- Possible space groups: look at the hkl conditions in the output list of the reviewing of the complete input data provided after a solution is found from the first N lines.
- Additional information on Dicvol06 should appeared in ref. 9.
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