Uniaxial Phase Transition in Si : Ab initio Calculations

TL;DR

This study uses ab initio calculations to analyze uniaxial phase transitions in silicon, revealing conditions under which different phases become stable and how phase transitions differ from hydrostatic compression.

Contribution

It provides a detailed phase diagram for silicon under uniaxial compression, highlighting the importance of directional stress in phase stability and transition pathways.

Findings

Stable phases under uniaxial compression are diamond, beta-tin, and sh.

Phase transitions can occur only under uniaxial stress, not hydrostatic.

Direct diamond-to-sh transition is possible when Px>Pz.

Abstract

Based on a previously proposed thermodynamic analysis, we study the relative stabilities of five Si phases under uniaxial compression using ab initio methods. The five phases are diamond, beta-tin, sh, sc, and hcp structures. The possible phase-transition patterns were investigated by considering the phase transitions between any two chosen phases of the five phases. By analyzing the different conributions to the relative pahse stability, we identified the most important factors in reducing the phase-transition pressures at uniaxial compression. We also show that it is possible to have phase transitions occur only when the phases are under uniaxial compression, in spite of no phase transition when under hydrostatic commpression. Taking all five phases into consideration, the phase diagram at uniaxial compression was constructed for pressures under 20 GPa. The stable phases were found to be diamond, beta-tin and sh structures, i.e. the same as those when under hydrostatic condition. According to the phase diagram, direct phase transition from the diamond to the sh phase is possible if the applied uniaxial pressures, on increasing, satisfy the condition of Px>Pz. Simiilarly, the sh-to-beta-tin transition on increeasing pressures is also possible if the applied uniaxial pressures are varied from the condition of Px>Pz, on which the phase of sh is stable, to that of Px<Pz, on which the beta-tin is stable.