Laboratory of Crystallography and Coordination Chemistry

National Centre for Scientific Research "Demokritos", Institute of Materials Science

Athens, Greece

 

 

X-ray Crystallography on Powder or Thin-Film Samples

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X-ray Crystallography on Powder or Thin-Film Samples

X-ray Crystallography on Single Crystals

X-ray Crystallography on Twinned Crystals

Development of XRD Instrumentation

Molecular Magnetism and Spectroscopy

Reactivity of Cluster Compounds 

Powder diffraction data are analyzed by applying the Rietveld method and the obtained structural information is used for the study of structural physical properties correlations. The method has been applied mainly by us for the study of hard-magnetic and superconducting materials. The correct space group of  the structure of the hard magnetic phase  Nd3 (Fe ,Ti)29 was assigned by our group as a result of Rietveld analysis on X-ray diffraction data obtained with a conventional source. The application of Rietveld-refinement on high-resolution Neutron Powder Diffraction data at room temperature for the parent and the nitrided compound give an unambiguously determination of the space group and the  calculated high-precision crystal structure parameters are  used for the calculation of the corresponding Wigner-Size  cell of  each crystallographic site. The calculation of the polyhedral configuration of each site which is based on the nearest neighbours is a useful tool for comparative studies of structurally similar compounds.

 
 
 

X-ray Crystallography on Single Crystals

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Besides routine crystal structure determinations we deal with disordered crystals. By analyzing the details of Fourier Maps it is possible to reveal the different orientations of disordered  ligands in clusters or the disorder positions of counter anions.

 
 
 

X-ray Crystallography on Twinned Crystals

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Based on the approach described by R. Sparks (www.hwi.buffalo.edu/ACA97/abstracts/text/E184) it is possible to solve and refine crystal structures with data collected with 4-circle diffractometers and from crystals which are  Non-Merohedral Twins. In this case the reciprocal lattices of the different twin domains do not superimpose exactly. The twinning can be detected from the reflection pattern (broad reflections, split reflections etc.) at the early stages of analysis (indexing).  The first (and main) problem is to index the recorded reflections, because they belong to two or more lattices from  two or more crystals of the same compound  and   the second is to find the Twin Law.

 
     

Development of XRD Instrumentation

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Silicon microstrip detectors have effectively and routinely used in high energy physics as a tool for tracking ionizing radiation. Silicon detectors built on high resistivity substrate of 1mm thickness, guarantee high absorption efficiency all the way up to 25 keV X-rays, where 40% absorption efficiency is assured. For substrates of 300μm thickness the efficiency is almost 100% at 8keV (Cukα radiation). From this point of view, Si is an appropriate material for X-ray crystallography. Different integrated detector systems (detecting plus VLSI amplification units), based on silicon microstrip technology have been tested at synchrotron radiation sources. Here we report on measurements with an integrated powder diffractometer operating with a conventional CuKα source, equipped with a Göbel mirror and a 32-channel silicon microstrip detector. The angular range covered by the 32 channels is 1.42°.  The X-ray diffraction spectra from different samples are recorded in steps of 1.0° or 1.4° of the detector.

 
     
 

Molecular Magnetism and Spectroscopy

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We are interested in the study of the electronic structures of coordination compounds through the combined use of magnetic (DC/AC magnetic susceptometry) and spectroscopic (EPR, 57Fe-Mössbauer) techniques.

Polynuclear complexes of paramagnetic ions exhibit complicated electronic structures due to magnetic exchange interactions (between different spins) and single-ion anisotropies (of a particular spin).

We try to map the energy spectra derived therefrom, and to ascertain the type, sign and size of the magnetic exchange interactions (isotropic, antisymmetric, anisotropic etc) and the single-ion anisotropies (axial, rhombic).

Single-Molecule Magnets are a type of complexes that interests us with respect to their magnetic properties. These molecules show slow magnetic relaxation at low temperatures, similar to that of classical magnets.

Magnetic chain compounds and rings exhibit interesting magnetic properties. Often the properties of the former can be approximated by those of the latter.

 
     
     
 

Reactivity of Cluster Compounds

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The reactivity of cluster compounds is an area of deep chemical interest, but difficult to study due to the labile nature of metal-ligand bonds and the multitude of products simultaneously present in solution.

The main questions asked are:

(a) what products occur from a particular reaction mixture, and how do these depend on various reaction parameters (solvent, counter ions, crystallization method, ligand sidegroups)?

(b) what is the reactivity of preformed clusters in solution, and what are the possible interconversions between clusters of the same reaction system?

The really fundamental question, though, is:

Is the Chemistry of polynuclear coordination complexes:

(i) under operator control, or

(ii) in the realm of serendipitous self-assembly?

Although for the time being the answer is definitely (i), we hope that answering questions (a) and (b) for many systems, it will slowly converge to (ii).

 
     
     
 

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