ADVANCES IN SINGLE-CRYSTAL HIGH-PRESSURE DIFFRACTION. G. Herrmannsdörfer, R.J. Angel, D.R. Allan and R.M. Miletich, Bayerisches Geoinstitut, Universitat Bayreuth, D-95440 Bayreuth, Germany

We have designed a new diamond anvil cell for X-ray diffraction that is suitable for determining both equations of state and structures of single crystals at high pressures. The cell design incorporates the best features of previous cell designs: massive supports to eliminate elastic deformation of the cell under load, widely spaced guide pins to ensure the maintenance of alignment, and diamond mounts that allow precise alignment of the diamonds. Tests of the cell with 600um culet diamonds have generated sample pressures in excess of 22 GPa on single-crystal samples.

In combination with the new diamond cell design we have developed the use of a large Huber 4-circle diffractometer specifically designed to optimise precision in cell-parameter measurement. The large size of the diffractometer leads to intrinsically better collimation than is available on smaller designs, and hence greater resolution in peak positions. In combination with the development of peak centring algorithms and the stability of the new DAC design, we can routinely achieve precision in unit-cell volume measurements of better than 1 part in 10,000 at high pressures. This allows us to determine equation of state parameters to much higher precision than previously achieved; typical precisions for materials with bulk moduli of ~80 GPa are 0.1% in K, and +/-0.3 in K'. These more precise measurements have also revealed the existence of previously unsuspected changes in the compression mechanisms of complex structures at high pressures.