SEMINARIS DE LA FACULTAT DE GEOLOGIA I L'INSTITUT DE CIÈNCIES DE LA TERRA 'JAUME ALMERA'
Karen Friese (Dpt. Física Materia Condensada, Universidad del País Vasco, País Basc)

A modulated structure is characterized by a basic structure in combination with a modulation or periodic perturbation. If the underlying periodicity of the basic structure and the periodicity of the modulation are in irrational relationship one speaks of incommensurately modulated structure. In reciprocal space the information about the basic structure is contained in the main Bragg reflections while the additional satellite reflections which are usually considerably weaker in intensity than the main Bragg reflections contain the main information about the modulation.

The occurrence of an incommensurate modulation leads to the breakdown of the three-dimensional periodicity of the underlying crystal lattice. However, as the modulation function itself is periodic, it is possible to resume the periodicity by adding one or more additional dimensions to the usual three-dimensional space and, this way, one can recover the periodicity in a higher-dimensional space. This so called superspace approach has been used successfully for the determination of aperiodic structures since its development in the 1970s and the implantation into various structure refinement programmes in the subsequent years.

Modulated structures are frequently observed in mineral systems though usually their existence is restricted to certain pressure/temperature conditions and often depends on the composition. During the presentation some examples of modulated mineral structures and the underlying reasons for the modulations will be presented.

Dia: 26 d'Octubre de 2011
Hora: 12:00
Lloc Sala d'actes de l'Institut Jaume Almera