Ab initio study of the beta$-tin->Imma->sh phase transitions in silicon and germanium

摘要

We have investigated the structural sequence of the high-pressure phases ofsilicon and germanium. We have focussed on the cd->beta-tin->Imma->sh phasetransitions. We have used the plane-wave pseudopotential approach to thedensity-functional theory implemented within the Vienna ab-initio simulationpackage (VASP). We have determined the equilibrium properties of each structureand the values of the critical parameters including a hysteresis effect at thephase transitions. The order of the phase transitions has been obtainedalternatively from the pressure dependence of the enthalpy and of the internalstructure parameters. The commonly used tangent construction is shown to bevery unreliable. Our calculations identify a first-order phase transition fromthe cd to the beta-tin and from the Imma to the sh phase, and they indicate thepossibility of a second-order phase-transition from the beta-tin to the Immaphase. Finally, we have derived the enthalpy barriers between the phases.