Pressure dependence of theImmaphase of silicon

A detailed structural study of the newly observed orthorhombic phase of silicon (space group Imma) has been made using angle-dispersive powder-diffraction techniques and an image-plate area detector. The Imma phase is found to be stable between 13. 2 (3) and 15. 6 (3) GPa, and both the -tin-to-Imma and Imma-to-simple-hexagonal transitions are found to be first order with volume changes (/V₀) of 0. 2 (1) % and 0. 5 (1) %, respectively. The volume discontinuities at the transitions are accompanied by pronounced discontinuities in, the atomic coordinate of the Imma phase, which is found to vary from 0. 3 to 0. 4 over the stability range of the Imma phase.