Abstract

The knowledge of the structure-property relationships is basic for the comprehension of the action mechanism of all materials. In particular, many solid-state compounds are available in the form of microcrystalline powder. In this case, the discovery of the crystal structure by diffraction methods is usually not easily achieved even in the case of a small molecule. Difficulties are met to attain a correct interpretation of the experimental X-ray powder diffraction pattern, due to peak overlap, incorrect background estimation, and possible preferred orientation effects. Modern instrumentation, theory and software are addressed to overcome such limits for accomplishing a successful crystal structure solution [1, 2]. Powder solution methods can be classified into two main groups: Reciprocal space methods, as Direct Methods (DM); Direct space methods, as Simulated Annealing (SA). They present different limits and advantages. The choice of the best strategy to adopt depends on some factors: experimental data quality, experimental resolution, peak overlap, structure complexity expressed in terms of number of non-hydrogen atoms in the asymmetric unit and/or number of degrees of freedom of the structure model, as well as available information on the expected molecular model. EXPO [3] is an advanced crystallographic software capable to perform the solution by DM or SA, and also to efficiently combine them [4]. The conditions requested for a profitable outcome by DM and SA, respectively, are discussed and compared by using examples. Useful suggestions, for preferring one method rather than the other one, are given and the opportunity of using both in EXPO is investigated.

Highlights

  • X-ray Powder Diffraction (XPD) is an effective analytical method for identifying and characterizing materials available in the form of microcrystalline powder, of different nature and scientific and technological interest

  • Diffraction peak overlap prevents from reliably estimating of experimental information derived from the powder pattern

  • Not always background can be described by a simple analytical function

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Summary

Polycrystalline materials

80 m good quality single crystal bad dimension single crystal (tenths of a millimetre). X-ray Powder Diffraction (XPD) is an effective analytical method for identifying and characterizing materials available in the form of microcrystalline powder, of different nature and scientific and technological interest (chemical, pharmaceutical, biological, mineralogical, for cultural heritage, ..). International Tables for Crystallography, Volume H, Powder Diffraction; C.J. Gilmore, J.A. Kaduk, H.

Powder diffraction
Structure solution by powder diffraction
The most widely used software
Extraction of the structure factor moduli
The structure model is imported in the unit cell
CF computation corresponding to the trial structure
Registration and Download of EXPO software

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