Polycrystalline Materials: Crystal Structure Solution in the Reciprocal Space or/and Direct Space?

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.

References

[1] Powder diffraction Theory and Practice; R.E. Dinnebier, S.J.L. Billinge Eds., The Royal Society of Chemistry, Cambridge, 2008.

[2] International Tables for Crystallography, Volume H, Powder Diffraction; C.J. Gilmore, J.A. Kaduk, H. Schenk Eds., Wiley: New York, 2019.

[3] A. Altomare, C. Cuocci, C. Giacovazzo, A. Moliterni, R. Rizzi, N. Corriero, A. Falcicchio. J. Appl. Cryst. 2013, 46, 1231.

[4] A. Altomare, C. Cuocci, N. Corriero, A. Falcicchio, R. Rizzi, Crystals 2019, 10(1), 16.