ACCURATE DETERMINATION OF STRUCTURAL PARAMETERS AT HIGH PRESSURES

Michael Hanfland

ESRF, B.P. 220, 38043 Grenoble, France

A high pressure diffraction beamline at a third generation synchrotron source like the ESRF provides enough flux to study almost any sample to megabar pressures in a diamond anvil cell. For example the angle dispersive powder diffraction set-up of the ID9 beamline delivers ~1x10¹¹ photons/s into a 30x30 mm² spot at 27 keV small enough to perform measurements to at least 60 GPa. For measurements at higher pressures the spot size can be reduced further without a substantial loss in flux. Image plates are used as detectors. Exposure times are in the order of minutes. Part of this immense gain in flux compared to first and second generation sources is utilised to improve the resolution by placing large image plates (area: 350 x 430 mm^2.) further away from the sample. The angular resolution at 450 mm sample to image plate distance is D2q = 0.04^o FWHM. Due to this high resolution solids with complex crystal structures can now be studied with the necessary accuracy.

Structural properties of molecular nitrogen were examined between 5 and 50 GPa. Diffraction pattern with considerably improved quality compared to previous measurements were collected. Structural parameters were refined. Values with an accuracy comparable to single crystal studies were obtained. A new tetragonal phase was discovered. Good agreement between measured and calculated diffraction pattern was achieved assuming, that the structural phase transition taking place at 11 GPa is due to an ordering of the disklike disordered molecules of the cubic d-phase. The transition to the e-phase occurred at 16.5 GPa. Its structure is rhombohedral (R-3c) as found earlier at lower temperatures and pressures and not tetragonal as suggested by Monte Carlo and Molecular Dynamics calculations.

Another system studied is LaFeO₃. Like other rare earth transition metal oxides LaFeO₃ crystallises in an orthorhombically distorted perovskite structure (Pbnm). At low pressures we are not only able to measure extremely accurate lattice parameters (Da/a ~ 10^-4) but also reliable atomic positions. Even the positions of the light oxygen atoms can bee determined with reasonable accuracy. This makes it possible to study changes in important structural parameters like the Fe-O(1)-Fe and Fe-O(2)-Fe angles with pressure, which are a measure of the size of the distortion and govern the electronic and magnetic behaviour. Above 10 GPa Laser annealing was necessary to reduce uniaxial pressure effects. Two structural phase transition were found, one at 20 GPa to a higher symmetric orthorhombic space group (probably Ibmm). The second one at 38 GPa to a tetragonal structure is related to a transition from antiferomagnetic to paramagnetic order of the iron atoms recently observed in a Mössbauer study.