PRESSURE-DEPENDENCE OF THE IMMA PHASE 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 beta-tin-to-Imma and Imma-to-simple-hexagonal transitions are found to be first order with volume changes (DELTAV/V0) of 0.2(1)% and 0.5(1)%, respectively. The volume discontinuities at the transitions are accompanied by pronounced discontinuities in DELTA, the atomic coordinate of the Imma phase, which is found to vary from approximately 0.3 to 0.4 over the stability range of the Imma phase.