## Lattice instability during solid-solid structural transformations under general applied stress tensor: example of Si I to Si II with metallization

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Density functional theory was employed to study the stress-strain behavior and elastic instabilities during the solid-solid phase transformation (PT) when subjected to a general stress tensor, as exemplified for semiconducting Si I and metallic Si II, where metallization precedes the PT, so stressed Si I can be a metal. The hydrostatic PT occurs at 76 GPa, while under uniaxial loading it is 11 GPa (3.7 GPa mean pressure), 21 times lower. The Si I to Si II PT is described by a critical value of phase-field’s modified transformation work, and the PT criterion has only two parameters given 6 independent stress elements. Our findings reveal novel, more practical synthesis routes for new or known high-pressure phases under predictable non-hydrostatic loading, where competition of instabilities can serve for phase selection, rather than free energy minima used for equilibrium processing.