Debvin

is offered to you by Sergio Brückner

Polymer Crystallography

Dipartimento di Scienze e Tecnologie Chimiche
Università di Udine, 33100-Udine, Italy

A program for Rietveld refinement with generalized coordinates subjected to geometrical restraints.

The crystallographic problem
Calculations
References
Download the program (513 KB)

The crystallographic problem

DEBVIN is a program designed to carry out Rietveld refinements with X-ray or neutron powder diffraction data. This program is a development of PREFIN, a program originally written by A. Immirzi (Immirzi, 1980), whose main feature is that of combining the Rietveld approach with the use of generalized coordinates.
Improvements present in DEBVIN are the result of experience accumulated in a number of years of applications, particularly in the field of polymer crystallography ( see for example Brückner, 1988, Meille, Brückner & Lando, 1989, Trifuoggi et al., 1994). In these cases, as structures are complex and diffraction patterns highly overlapped and confined to low diffraction angles, the use of generalized coordinates (g.c.) instead of fractional coordinates (f.c.) becomes a decisive tool as it considerably reduces the number of refinable parameters. The most relevant innovations in this part of the program are (i) the possibility of handling any structural model without performing (as was necessary with PREFIN) the implementation of an 'ad hoc' written subroutine, and (ii) the possibility of further reducing the degrees of freedom by introducing restraints among the g.c. At present, therefore, no programming experience is required by the user and DEBVIN can be distributed in its executable version without requiring a new compilation for each specific problem under investigation. Another important innovation is the possibility of refining the average dimensions of crystallites along directions parallel to the unit cell axes. In the case of polymers, but also in other cases, crystallites (lamellae) present a strong dimensional anisotropy that determines peak widths, which depend not only on the scattering angle q but also on Bragg indices hkl. It is to be noted however that the dimensions refined in this way have a precise physical meaning only in the case of strictly Lorentzian peak shapes and, even in this case, they are only an underestimate of the real dimensions unless instrumental broadening is taken into account. When these conditions are not met, the numerical values obtained are to be handled with caution, but nevertheless they usually allow for a significant reduction of the disagreement between the observed and the calculated profiles that, otherwise, owing to correlation effects, coul disturb the refinement of other parameters.

Calculations

Very briefly , DEBVIN performs the following tasks:
i) Choice of Pearson VII or pseudo-Voigt profile function.
ii) Peak asymmetry accounted for by splitting the peak into two half-peaks with different and refinable full widths at hal maximum.
iii) Adjustment of lattice constants, preferred orientation, zero point, scale factor, isotropic thermal parameters.
iv) Use of fixed (neutrons) as well as variable (X-ray) atomic scattering factors.
v) Automatic generation of the list of contributing reflections with proper multiplicities.
vi) Fixed or variable spacing in profile sampling.
vii) Presence of Ka1 and Ka2 splitting.
viii) Anisotropy of crystallite average dimensions.
ix) Presence of a second phase - in this case the program can perform the crystallographic refinement of one phase provided the other is known.
x) Presence of background intensity, which can be represented either by the properly scaled contribution of a profile recorded from a totally amorphous sample or by one or more computed and refined bell-shaped curves.
xi) Use of generalized coordinates to generate the structural model and possibility of introducing properly weighted restraints among them.
xii) Plot on the screen and, optionally, on a laser writer of both the structural model and of the results of the refinement in the form of observed + calculated profiles, background contribution and difference profile.

References

1) Immirzi, A. (1980). Acta Cryst. B36, 2378-2385.
2) Brückner, S. (1988). Chim. Ind. 70, 48-53.
3) Meille, S.V., Brückner, S. & Lando, J.B. (1989). Polymer, 30, 786-792.
4) Trifuoggi, M., De Rosa, C., Auriemma, F., Corradini, P. & Brückner, S. (1994).
Macromolecules, 27, 3553-3559.

The best citation for DEBVIN is:

Sergio Brückner & Attilio Immirzi, J. Appl. Cryst. (1997), 30, 207-208.

Please send your comments and your suggestions to Sergio Brückner .