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Crystallographic Methodologies

A number of Italian research groups are actively involved in the development of crystallographic methodologies, producing relevant results both in the field of structure determination and characterization of materials and investigation of new experimental techniques. For each group, the main research areas are summarized.

In Bari and Rome [1], [2] the main research topic in the last thirty years has been the theory and the application of the phase problem in crystallography (in collaboration with researchers belonging to the University of Perugia), producing significant scientific contributions at international level. The research aims at carrying out innovative procedures for ab-initio crystal structure solution. The methods, so far developed, have been implemented in computer programs largely used by the crystallographic scientific community all over the world: Sir97 (small molecules), the heir of Sir92 and CAOS (least squares refinement); EXPO (powder data); Sir2002 (proteins). A very recent methodological activity concerns the crystallographic characterization of materials (quasicrystals, nanocrystals) and the quantitative analysis of multicomponent polycrystalline mixtures (program QUANTO).

In Como [3] and Milan [4] the research interests are the structural determination by ab-initio powder diffraction methods of covalent inorganic and organometallic polymers and the structure-property relations in functional materials (transition-metal-based molecular magnets; electroluminescent materials, organometallic catalysts). The most recent activity is focused on the determination of the local structure of disordered materials by total scattering analysis with application to amorphous or paracrystalline metal-coordination polymers.

foto 5
The unique formation of a crystal phase containing cyclic oligomers and helical polymers of the same monomeric fragment as discovered by ab-initio XRPD studies on conventional laboratory data N. Masciocchi, G.A. Ardizzoia, G. La Monica, A. Maspero, A. Sironi, Angew.Chem., Int. Ed. Engl., (1998), 37, 3366

In Parma [5] and Bologna [6] the Transmission Electron Microscopy (TEM) techniques, in particular High Resolution Electron Microscopy (HREM) and Electron Diffraction (ED), have been (and are still) used to support in a strategic way the conventional diffraction techniques for the solution of complex structural problems. More recently, with the enlargement of the research group [7] a large effort has been concentrated on the ab-initio crystal structure solution by ED data. Three main research lines are developed: 1) the experimental reduction of dynamical effects on ED intensities (implementation of a beam-precession technique on a Philips CM30T microscope), 2) the development of computer programs for quantitative treatment of ED data (the package QED), 3) the characterization of strategies for direct methods application to ED data and synergetic use of HREM, ED, X-ray and neutron powder diffraction data in the structure determination of materials.

The group of Pisa [8] is interested in X-ray diffractometry methods. In particular, the research subjects are: technological aspects, production of prototypes, standardization and metrological aspects, new approaches to Round Robin designs.

In Pavia [9], [10], Milan [11] and Turin [12] the scientific activity aims at improving some crystallographic procedures for the characterization of materials. Researches on least-squares structure refinements, electron density reconstruction, Leverage analysis and Maximum Entropy Method are in progress.

In Salerno [13] the research activity is devoted to the study of stereoregular crystalline polymers from powder and/or fiber samples with particular attention to the solution and refinement of polymer structures (the program TRY has been developed) and to the indexing and extraction of integrated intesities from image plate detectors.

In Siena [14], the main research lines are: the improvement of the Gandolfi technique to minimize the number of missing reflections and obtain reliable “powder” diffraction patterns from a single grain as small as 20m m diameter; the quantitative determination of the diffraction aspect in pseudosymmetric crystal structures.

In Milan [15], the research is concerned with the development of software for i) electron density distributions analysis, including topological and electrostatic properties (TOPOND, TOPXD and PAMoc), ii) lattice dynamics (VIBR) and iii) crystal structure and polymorphism’s prediction (PROMET).

foto 5
Gradient field of the electron density in the molecular plane of urea, in gas phase (left) and in the crystal (right), obtained using the TOPOND Code (C. Gatti, V.R. Saunders, C. Roetti , J. Chem. Phys. 101, 1994,10686-10696).

The group in Turin [16] studies the electronic structure and properties of perfect and defective crystalline materials. They develop ab initio methods implemented in a computational code, CRYSTAL, that is distributed to the scientific community and is applied to the study of physical, chemical, electric and magnetic properties of ionic, covalent and molecular crystals.

[1] IC-CNR Sede di Bari, Sezione di Roma, carmelo.giacovazzo@ic.cnr.it

[2] Dip. Geomineralogico, Università di Bari, carmelo.giacovazzo@ic.cnr.it

[3] Dip. di Scienze Chimiche, Fisiche e Matematiche, Università dell’Insubria, Como, norberto.masciocchi@uninsubria.it

[4] Dip. di Chimica Strutturale e Stereochimica Inorganica, Università di Milano, angelo.sironi@istm.cnr.it

[5] Dip. di Chimica Generale ed Inorganica, Chimica Analitica e Chimica Fisica, Università di Parma, calestg@ipruniv.cce.unipr.it

[6] IMM-CNR, Sezione di Bologna, migliori@lamel.bo.cnr.it

[7] Dip. Scienze della Terra, Università di Milano, mauro.gemmi@unimi.it

[8] Dip. di Scienze della Terra, Università di Pisa, berti@dst.unipi.it

[9] IGG-CNR, Sezione di Pavia, merli@crystal.unipv.it

[10] Centro Grandi Strumenti, Università di Pavia, boiocchi@crystal.unipv.it

[11] Dip. di Scienze della Terra, Università di Milano, Alessandro.Pavese@unimi.it

[12] Dip. di Scienze Mineralogiche e Petrologiche, Università di Torino, mauro.prencipe@unito.it

[13] Dip. di Chimica, Università di Salerno, immirzi@unisa.it

[14] Dip di Scienze della Terra, Universita’ di Siena, gregor@unisi.it

[15] ISTM-CNR, Milano, c.gatti@istm.cnr.it

[16] Dip. di Chimica, Università di Torino, roberto.orlando@unito.it