Phonons in the beta-tin, Imma, and sh phases of Silicon from ab initio calculations

TL;DR

This paper uses ab initio calculations to reinterpret Raman data of silicon's high-pressure phases, revealing earlier phase transitions and accurately predicting phonon frequencies and transition pressures.

Contribution

It provides a new interpretation of experimental data, identifying phase transitions in silicon and calculating phonon dispersion for multiple phases using density-functional theory.

Findings

Reinterpreted Raman frequencies indicating earlier phase transitions.

Calculated phonon dispersion curves matching experimental data.

Predicted transition pressures and soft modes in phase transitions.

Abstract

We present a new interpretation of measured Raman frequencies of a high-pressure structure of Silicon which was assigned previously to the beta-tin phase. Our results show that the beta-tin->Imma->sh phase transitions have been already indicated in this experiment which was performed before the discovery of the Imma phase. We have calculated phonon-dispersion curves for the beta-tin, Imma, and sh phases of silicon using the plane-wave pseudopotential approach to the density-functional theory and the density-functional perturbation theory within the local density approximation. With the new assignment, the calculated phonon frequencies display an excellent agreement with the experimental data, and can be also used to determine precisely the transition pressure for the Imma->beta-tin phase transition. The sh->Imma transition is accompanied by soft modes.