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Nontrivial Nanostructure Drives the Si-I to Si-II Transformation at High Pressures

Shifts of Si-I/Si-II interfaces as observed by in situ real-time Laue diffraction and MD simulation.

A recently published work that combines molecular dynamics simulations, crystallographic theory generalized for strained crystals, and in-situ real-time Laue x-ray diffraction, reveals unprecedented nanostructure evolution during pressure-induced Si-I→Si-II phase transformation. The research effort was led by a long-time HPCAT user Professor V. Levitas, (Anson Marston Distinguished Professor in Engineering/Vance Coffman Faculty Chair Professor in Aerospace Engineering at ISU and staff member at Ames National Laboratory) and his former Ph.D. student Hao Chen (now a Professor at East China University of Science and Technology, Shanghai, China). The collaborative effort with HPCAT-colleagues, Dmitry Popov and Nenad Velisavljevic, was recently published in Nature Communications in the paper, “Nontrivial nanostructure, stress relaxation mechanisms, and crystallography for pressure-induced Si-I → Si-II phase transformation”. https://doi.org/10.1038/s41467-022-28604-1

Due to large and highly anisotropic transformation strains, the observed nanostructure at the transition in silicon does not obey the equations of the classical crystallographic theory. Thus, instead of minimizing lattice incompatibility and elastic energy at the unique irrational interface between semiconducting Si-I and twin-related variants of the metallic Si-II, unexpected nanobands, consisting of alternating strongly deformed and rotated residual Si-I and the third variant of Si-II, form {111}  interface with Si-I. The new basic principle that governs such counterintuitive nanobands and interfaces is to produce an almost self-accommodated nanostructure that minimizes elastic energy of the long-range stresses in bulk, tolerating high short-range stresses at complex interfaces which are necessary to keep residual Si-I and reduce the resultant volume discontinuity. The interfacial bands arrest the {111}  interfaces, leading to repeating the nucleation-growth-arrest process. This in turn results in a growth of the new phase by propagating (110)  interface. Traditional crystallographic theory would not have predicted either of the two processes.

High-pressure Laue diffraction and molecular dynamics simulations have completely different spatial and time resolutions, and generally would not correlate with each other. However, this research shows major agreement between these two approaches for the observed counterintuitive nanostructure. Such in-situ experimental measurements coupled with molecular dynamics, and theoretical approaches open opportunities to study other high-pressure transformations in Si and Ge, C, BN, and BCN systems and suggest revisiting other transitions involving high strain which have been studied using modern phase-field approaches to account for nanostructures that may have been ignored.

Shifts of Si-I/Si-II interfaces as observed by in situ real-time Laue diffraction and MD simulation. a Maps of 202 reflection from a Si-I crystal (left column) and maps of a diffuse reflection from Si-II (right column). More maps are available in the Supplementary Movie 1. b 3D schematic of the shifts of the Si-I/Si-II interfaces. c Simulated shifts of the Si-I/Si-II interfaces. In all the images the interfaces before the shifts are shown by dark blue and the interfaces after the shifts are shown by red lines; solid lines denote currently existing interfaces and dotted dashed lines denote interfaces existing in different states; directions of the shifts of the interfaces are shown by red arrows.

The editors at Nature Communications have featured this article on their Editors’ Highlights pagehttps://www.nature.com/collections/eecgdgijhh 

Chen, H., Levitas, V.I., Popov, D. et al. Nontrivial nanostructure, stress relaxation mechanisms, and crystallography for pressure-induced Si-I → Si-II phase transformation. Nat Commun 13, 982 (2022). https://doi.org/10.1038/s41467-022-28604-1

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