High pressure properties of rare earth intermetallics studied by synchrotron powder diffraction and ab-initio calculations

The result of this research project supported by the FWF is that there is now a working group at the TU Vienna established which has the experimental facilities and the knowledge for performing high pressure x-ray diffraction experiments on intermetallic systems and to analyse the results with respect to either the phase instability or the compressibility. One of the most important properties of the solid state is the crystal structure, i.e. the arrangement of the atoms inside the unit cell, since most of the physical properties depend strongly on the crystal structure. Normally the crystal structure of a compound is given at room temperature and ambient pressure. However there are many investigations known, which have shown that the crystal structure can change when the temperature or the pressure changes. Intimately related with the crystal structure are the physical properties of an intermetallic compound. In order to study the stability of a given crystal structure, pressure experiments with either a X-ray- or neutron - source have to be done. The goal of this project was to establish the conditions for performing pressure experiments on one of the European synchrotron sources. For this purpose a diamond pressure cell for pressures up to 20 GPa (200 kbar) has been constructed and successfully adapted for measurements at the ELETTRA synchrotron source in Trieste. A series of experiments have also been performed at ESRF (Grenoble) and at DESY (Hamburg). It appeared that the co-operation of the working group of the co-proposer Prof. Hafner and the project co-worker Dr. Lindbaum with respect to a theoretical explanation of the measurements was very successful. In many cases phase instabilities have been found experimentally which where theoretically predicted from ab-initio calculations. However, not only the phase stability of compounds has been studied in the scope of this project but also a number of compressibility measurements have been performed. One of the most interesting observations was, that manganese based compounds show a much stronger compressibility in comparison to other transition metal (such as iron, nickel or cobalt) based compounds.