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 {β}-tin-to-Imma and Imma-to-simple-hexagonal transitions are found to be first order with volume changes ({Δ}V/V₀) of 0.2(1)% and 0.5(1)%, respectively. The volume discontinuities at the transitions are accompanied by pronounced discontinuities in {Δ}, 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.