ESF Workshop

Properties of inorganic crystals from theory and experiment



Proposal

The proposed workshop is planned to give the opportunity to discuss the state-of-the-art of crystallographic and crystal chemical investigation of small inorganic compounds, exclusive of metals and semiconductors. The emphasis will be put on discussing studies of the structure and properties of compounds with unit cell volumes of less than about 500 \AA$^3$. For such relatively small structures, high quality experimental data can generally be obtained. The structure determination of these compounds from either single crystals or powders by x-ray or neutron diffraction is generally straightforward. In comparison to the evaluation of the diffraction data, the interpretation of spectroscopic data, which is typically used to further understand properties, such as bond strengths, is generally much more involved. This is especially true for low symmetry structures, and in those cases where no single crystals can be grown. Here, ab initio approaches can, at least in principle, provide complementary information and help resolve ambiguities in the interpretation of spectroscopic data.

During the workshop, we would like to discuss several aspects of the typical experimental approaches and how ab initio calculations can be used as a crystallographic tool. Small inorganic compounds are of interest to crystallographers from various scientific communities, such as mineralogists (where silicates are at the center of interest), inorganic chemists (where often molecular crystals are studied), material scientists (who are interested in properties of components of ceramics) and physicists (where the dynamics of crystals are often investigated). The experimental approaches include a large variety of spectroscopic techniques, such as nuclear magnetic resonance and M\"o\ss{}bauer spectroscopy (for the determination of electric field gradients and quadrupole interactions), optical spectroscopy in the visible (electronic transitions) and infrared (phonons), neutron scattering (phonon dispersion and crystal field transitions), etc.. Also, the crystallography of surfaces, grain boundaries and interfaces, as observed by TEM and STM/AFM is of current interest. The accuracy achieved in the experimental determination of electron density distributions by diffraction methods has significantly advanced in the last years, and a comparison of the experimental and theoretical limits will be very worthwhile.

Generally speaking, what would be very useful now is an assessment on a broad basis for which crystallographic experiments current state-of-the-art ab initio approaches can give results with `experimental accuracy', i.e.\ accurate enough to be useful in the interpretation and prediction of data. The theoretical discussion will be centered on the capapbilities of modern DFT-based programs, such as the FP-LAPW program WIEN97 and the plane wave pseudopotential program CASTEP, but of course advantages and disadvantages of other approaches will be evaluated as well. From the organisers point of view, it is important to discuss now, when several mature codes are available, which further developments are needed and where ab initio calculations can already reliably be used to complement measurements.


Motivation and objectives

The objective of the workshop is to bring together experimentalists and theoreticians from the different fields of crystallography, namely mineralogy, solid state chemistry, materials science and solid state physics.

We have obtained tentative confirmations for participation by experimentalists working in the fields mentioned above, namely phonon dispersion curves, determination of precise electron density distributions, STM and AFM measurements on surfaces, TEM studies of polytypes and defects, high pressure neutron and x-ray diffraction, optical spectroscopy, etc. From the theoretical side, there will be code developers and users with very different backgrounds, including materials science, theoretical chemistry, and mineralogy, who are all interested in ab initio studies of crystallographic aspects of inorganic compounds.

One motivation of the workshop is to mutually educate members from the different communities and spread knowledge of what is currently attainable experimentally and theoretically. This, from our point of view, has been successfully achieved in our 1997 workshop on mineralogical problems and hence we would like to employ the same approach with the emphasis on crystallography instead of mineralogy.

This workshop will be funded mainly by the ESF It will be co-sponsored by the German Crystallographic Society (Working group for computer experiments) and Molecular Simulations Inc (MSI). As in the 1997 workshop, each participant will be asked to submit an extended abstract, which will be made available about a month before the meeting. Also, the WWW will be used to prepare the meeting.

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