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Density functional theory (DFT) was employed to study the stress-strainbehavior, elastic instabilities, and metallization during a solid-solid phasetransformation (PT) between semiconducting Si I (cubic A4) and metallic Si II(tetragonal A5 structure) when subjected to a general stress tensor. Withnormal 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 independentstresses that drive a lattice instability for the Si I$\rightarrow$Si II PT,and a semiconductor-metal electronic transition. Metallization precedes thestructural PT, hence, a stressed Si I can be a metal. Surprisingly, astress-free Si II is metastable in DFT. Notably, the PT for hydrostaticpressures is at 75.81 GPa, while under uniaxial stress it is 11.03 GPa (or 3.68GPa mean pressure). Our key result: The Si I -> Si II PT is described by acritical value of the modified transformation work, as found with a phase-fieldmethod, and the PT criterion has only two parameters for a general appliedstress. More generally, our findings are crucial for revealing novel (and moreeconomic) material synthesis routes for new or known high-pressure phases undercontrolled and predictable non-hydrostatic loading and plastic deformation.

physical review letters

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