Abstract
The Imma phase of Si is found by first-principles calculations to have unstable states at the two ends of its structural range. These instabilities force first-order transitions to both the/3-Sn and the simple hexagonal phase. These transitions occur for the non-vibrating crystal at 96 and 110kbar, respectively. Two special computational procedures are described that have been developed to find the instabilities. Copyright © EPLA, 2010.