EXPO2004, the updated version of EXPO (Altomare, Burla, Camalli, Carrozzini, Cascarano, Giacovazzo, Guagliardi, Moliterni, Polidori & Rizzi, 1999) is able to solve crystal structures from unindexed powder diffraction pattern. It is more than the integration of EXTRA (Altomare, Burla, Cascarano, Giacovazzo, Guagliardi, Moliterni & Polidori, 1995) and SIRPOW (Altomare, Cascarano, Giacovazzo, Guagliardi, Burla, Polidori & Camalli, 1994). Its main steps are:
Indexing
An automatic peak search routine generates a list of "d" values corresponding to the experimental peaks. The values are processed by N-TREOR program (Altomare, Giacovazzo, Guagliardi, Moliterni, Rizzi & Werner, 2000) for providing the cell parameters.
Space Group Determination
A statistical analysis of the integrated intensities extracted by the pattern according to the Laue class(es) is performed. It is able to provide a list of possible space groups, ranked according to a figure of merit. The user may graphically select the suitable choice.
Extraction
The procedure of decomposing the pattern for extracting the integrated intensities uses the Le Bail approach combined with least squares cycles (as in EXTRA package). Then structure factor modulus value is calculated for each reflection generated according to the cell parameters, the space group and the experimental pattern.
Direct Methods
This step is carried out as in SIRPOW package. The representation theory, proposed by Giacovazzo (1977, 1980) allows the derivation of powerful methods for estimating the phases of the structure factors. The mathematical approach makes full use of the space group symmetry. The process uses the integrated intensities obtained in the extraction step or can manage an external reflection file. It supplies a Fourier map which is chemically interpreted.
The structure model is suitably improved by an automatic approach combining Fourier map calculations with preliminary least squares cycles .
Structure Model Refinement
The structure model is optimized by Rietveld method (Rietveld, 1969).
EXPO2004 works in a completely automatic way, from indexing to structure determination. Alternatively, not default options can be chosen. The improved user-friendly graphic interface guides the user so that people not experienced in Crystallography can solve the structure also.
Each step can be carried out alone. For example, it is possible to apply Direct Methods avoiding the extraction process, but an external reflections file must be supplied.
Several prior information-based strategies can be adopted for improving the pattern decomposition in order to optimise the structure factor estimates.
EXPO2004 creates the following
files (in the work directory):
- a direct access file to preserve data and results (.bin),
- an ascii file containing (h k l) and integrated intensities of the generated
reflections (extra.hkl)
- an ascii input file for Sirpow (extra.sir),
- an ascii file to store peak positions if the peak search procedure is applied
via directive (ntreor.pea)