Deformation, lattice instability, and metallization during solid-solid structural transformations under general applied stress tensor: example of Si I -> Si II

TL;DR

This study uses density functional theory to analyze how general applied stresses induce phase transformation, lattice instability, and metallization in silicon, revealing new insights into stress-driven phase behavior and synthesis routes.

Contribution

It introduces a phase-field based criterion for predicting Si I to Si II transformation under arbitrary stress conditions with only two parameters.

Findings

Metallization occurs before structural transformation under stress.

Critical stress combinations induce lattice instability and phase transition.

Transformation work criterion effectively predicts phase change under complex stress states.

Abstract

Density functional theory (DFT) was employed to study the stress-strain behavior, elastic instabilities, and metallization during a solid-solid phase transformation (PT) between semiconducting Si I (cubic A4) and metallic Si II (tetragonal A5 structure) when subjected to a general stress tensor. With normal stresses ($\sigma_1$, $\sigma_2$, $\sigma_3$) acting along $\left<110 \right>$, $\left<1\bar{1}0 \right>$, and $\left<001\right>$, respectively, dictating the simulation cell, we determine combinations of 6 independent stresses that drive a lattice instability for the Si I$\rightarrow$Si II PT, and a semiconductor-metal electronic transition. Metallization precedes the structural PT, hence, a stressed Si I can be a metal. Surprisingly, a stress-free Si II is metastable in DFT. Notably, the PT for hydrostatic pressures is at 75.81 GPa, while under uniaxial stress it is 11.03 GPa (or 3.68 GPa mean pressure). Our key result: The Si I -> Si II PT is described by a critical value of the modified transformation work, as found with a phase-field method, and the PT criterion has only two parameters for a general applied stress. More generally, our findings are crucial for revealing novel (and more economic) material synthesis routes for new or known high-pressure phases under controlled and predictable non-hydrostatic loading and plastic deformation.