HIGH-PRESSURE SINGLE-CRYSTAL X-RAY DIFFRACTION STUDY OF [[epsilon]]-FeSi. Ross, N. L. Department of Geological Sciences, University College London, Gower St, London WC1E 6BT, UK

The crystal structure of cubic iron monosilicide, [[epsilon]]-FeSi, has been determined between 0 and 7 GPa using single-crystal X-ray diffraction. At all pressures studied, the material has space group P2₁3, with Z = 4 and both atoms on sites 4a, (x,x,x). The unusual seven-fold coordination of both Si and Fe is retained at high pressure, and the positional coordinates of Fe, x_Fe = 0.137, and Si, x_si, = -0.158, do not vary significantly with pressure. The interatomic distances show an approximate linear decrease with pressure due the reduction of the cell parameter. The bulk modulus, K_o, determined from fitting a third-order Birch-Murnaghan EOS to the pressure-volume data is 176(3) GPa (with K_o' = 4.0). The results from this study will be compared with recent high-pressure time-of-flight neutron powder diffraction experiments of [[epsilon]]-FeSi using the POLARIS diffractometer at the ISIS spallation neutron source (Wood et al., J. Phys.: Condens. Matter 7, L475-L479, 1995).