X-Ray and Neutron Dynamical Diffraction - Theory and Applications,

Erice, 9-21 April 1996. A Report.

a NATO Advanced Study Institute

sponsored by the International Union of Crystallography and the European Commission, DG XII

 

The 23rd crystallographic course at the Ettore Majorana Centre, Erice, Sicily, Italy, was run as a Nato Advanced Study Institute with A. Authier (Paris) and S. Lagomarsino (Rome) as co-Directors, P. Spadon (Padova) and L. Riva di Sanseverino (Bologna) as local organizers, R. Colella (Purdue) and B. Tanner (Durham) being the other members of the organizing committee.

The main reason for planning the course is the strong revival of the applications of dynamical diffraction which are brought about by the Synchrotron Radiation (SR) and, in particular by the possibilities offered by the third generation SR sources.

About one hundred participants from twenty four different countries took part under Paola's and Lodovico's watchful eyes. They listened to 48 main lectures (judged too many in the questionnaire), discussed 42 posters during two poster sessions, worked out several exercises during tutorials and tried computer programs brought by the invited speakers. The leisure time (judged "too limited" by a significant number of participants) included two half-day archaeological excursions, dry and sunny weather (!!!!) throughout the full duration and evening Marsala wine tasting......

The photo shows along the first row Lodovico, ,, Stefano Lagomarsino, Bob Batterman,
Paola, and along the second row Henk Schenk, six to be nominated, Andre Authier, .., Manuela
Panzalorto, .Little bit to Lodovico's left, third row, the face of Norio Kato, a IUCr President.

The first part of the course reviewed the basic principles of the dynamical diffraction of x-rays and neutrons by perfect and nearly perfect crystals with special attention to the hughly asymmetrical cases, polarization of the x-rays and various types of polarizers for x-rays, statistical theory for highly imperfect crystals. A great importance was given to the properties of SR and the optical devices used in theoretical and experimental aspects of magnetic and resonant nuclear scattering of x-rays was also described.

The second part considered the principles of the various techniques used in the modern applications of dynamical diffraction which are very varied in nature. Several contributions were devoted to x-ray and neutron diffraction topography which enables the direct imagingof crystal defects. The various settings were described and the new possibilities opened by the third generation SR sources were pointed out. The theoretical interpretation of contrast was given for various types of defects.

The main applications presented include the characterization of of high technology materials , the in situ study of crystal growth and the relation of crystal defects to the growth conditions, in situ study of plastic deformations and phase transitions, the analysis of the distribution and shape of domains in magnetic materials. Another technique for the characteriation of defects in imperfect materials and the analysis of strains is the high resolution diffraction of x-rays. The theory of reciprocal space mapping was presented in detail and examples of strain-analysis were given in the case of III-V multilayer compounds and ion-implanted silicon. The third type of applications concerned the location of impurity atoms at crystal surfaces or interfaces by means of the fluorescence emitted at x-ray standing waves antinodes. The same technique is applied also in the study of this films or long period structures.

Multiple beam diffraction occurs very often in electron diffraction and is usually avoided in x-ray diffraction. The theory for x-rays is complicated by the fact that electromagnetic waves are vector waves while the wave associated to an electron or a neutron beam is scalar. New developments on the analysis of the n-beam diffraction of x-rays were presented and it was shown that from diffraction profiles in the neighbohrood of three-beam diffraction it is possible to determine the absolute phase of the structure factors. This is very important and will help solving crystal structures even for macromolecules such as small proteins.

Finally, the principles of x-ray and neutron interferometry were described and very promising applications in the fields of phase contrast microscopy and metrology in the nanometre regime were presented.

The following speakers were present: J. Baruchel, Grenoble; B.W. Batterman, Berkeley; M. Blume, Upton; K. Bowen, Coventry; R. Colella, Purdue; P. Fewster, Redhill; C. Giles, Campinas; V. Holy, Brno; K. Hummer, Karlsruhe; N. Kato, Nagoya; H. Klapper, Bonn; J. Patel, S. Diego; M. Schlenker, Grenoble; M. Servidori, Bologna; B.K. Tanner, Durham.

The "Vaciago Award", intended to honour the memory of an undefatigable supporter of italian young promising crystallographers, could not be assigned as the designation by referees converged on a very qualified and dynamical scientist, whose age trespassed the limit traditionally fixed.

A book will shortly be published by Plenum, containing the full texts of the lectures, edited by A. Authier with the collaboration of the members of the organizing committee. There are a few copies of the lecture notes left: they may be obtained by asking the Executive Secretary of the International School of Crystallography, Paola Spadon, Dept of Organic Chemistry, via Marzolo 1, 35131 Padova, Italy, fax +39 049 827 5239, email: paola@chor.unipd.it


Posted 24 June 1996, from a report written by A. Authier, slightly amended and integrated - also with a group photo (address data corrected Nov 27, 2000) by

Lodovico Riva di Sanseverino, fax +390 51 2094904,

Email: riva@geomin.unibo.it
after 2007 : lodovico.riva@unibo.it