Project

Discipline

Crystallography; Disordered structures; Diffuse scattering; Monte Carlo simulations

Lead
Lay summary
Modern technology demands ever more sophisticated materials. Knowledge of their atomic structure - a prerequisite for an understanding of their function and properties ­ is accessible through X-ray crystallography, but perfect lattice periodicity is usually necessary. Local deviations from perfect periodicity are often the cause of interesting physical properties, but hinder the routine crystallographic analysis, so that special methods of analysis are required for a full understanding of the molecular architecture.  Diffuse scattering is observed in an X-ray diffraction pattern if a crystal of a material shows deviations from peroidicity (disorder) and this project aims to measure and interpret diffuse scattering from disordered molecular materials with the aim of (1) contributing readily accessible, methodological advances to a part of crystal structure analysis which, unlike studies of ordered materials, is far from routine, and (2) adding investigations dealing with important chemicals to the collection of case studies of diffuse scattering.  Some of the work in this project will focus on the elucidation of the complex local structure of crystalline sodium tetrafluorolutetate, which shows the ability to change the colour (wavelength) of light from red to green or from green to blue. Other interesting materials exhibiting complex disorder may also be included in the study.

Direct link to Lay Summary

Last update: 21.02.2013

Active participation

Number	Title	Start	Funding scheme

The Determination of Disordered Crystal Structures from Diffuse ScatteringOur interest is in measuring and interpreting diffuse scattering from disordered materials with the aim of (1) further developing the use of recently established methods of analyzing diffuse scattering from molecular compounds, namely genetic algorithms, and (2) adding investigations dealing with important chemicals to the collection of case studies of diffuse scattering. The number of such studies is very, very limited compared with that for ordered structures (well over 500'000 compounds). In addition to gaining a better understanding of the structure and properties of the materials under investigation, the research is intended to contribute readily accessible, methodological advances to a part of crystal structure analysis which, unlike studies of ordered materials, is far from routine. Understanding disorder in materials is of interest internationally in the chemical and materials industries, because it is not infrequent that the crystal structures of materials which show scientifically interesting and potentially useful properties are also frustratingly disordered, yet related materials that are not disordered often do not show the desired properties.This is a follow-on proposal to the previous SNF project [SNF 200021-116363 (3 years) and 200020-129716 (one year extension) - The determination of disordered crystal structures from diffuse scattering]. The disordered material chosen for study in the precursor project, "cubic" a-NaLuF4, turned out to be much more complex than could be foreseen. We had anticipated that the average structure would be routinely determined from the Bragg reflections and that attention could then be focussed on the interpretation and modelling of the diffuse scattering, as was reported for another phase of the material, ß-NaLuF4. However, the diffraction pattern of the chosen phase of a-NaLuF4 contains nearly all crystallographic features imaginable: 6-fold twinning and commensurate modulation, in addition to diffuse scattering. Overcoming the enormous challenges presented by the data to just establish the average structure consumed all of the time in the precursor project. An interpretation of the diffuse scattering still remains to be undertaken and this proposal lays out our plans to address that aspect, thereby completing the original aims of the study.There is still much to be done to complete the investigation of a-NaLuF4 and the analysis of the diffuse scattering of the samples will form the backbone of the proposed project. We have good sets of synchrotron data for two phases of a-NaLuF4, and have established the average structures. The diffuse scattering information must first be extracted from the diffraction images (software available). We will make a qualitative estimate of the structural properties leading to the observed diffuse signals, and then tackle a quantitative analysis of the complex diffuse scattering using the Monte Carlo simulation and genetic algorithm approach. It is anticipated that the sequence of specific occupation of Na+/Lu3+ sites within and between columns along the long axis in the structure may play a significant role in the nature of the diffuse scattering, as was found for ß-NaLuF4.As part of a currently running SNF-funded Sinergia project at the University of Zurich, of which the applicant is a co-PI (CRSIK0_122706: Structure elucidation of disordered crystals from diffuse scattering, crystal modelling and supercomputing), a new software package for modelling disorder and diffuse scattering is being developed and will enter testing phase on real problems within the next few months (Zurich Oak-Ridge Disorder Simulation software - ZODS). An integral part of the project being proposed here will involve using and testing this new software in order to perform the desired modelling, so the two projects are closely interrelated and benefit each other.