Silicon allotropes by large-volume high-pressure techniques: crystal growth mechanisms, phase diagrams and hexagonal nanostructured Si-6H by in situ X-ray diffraction and computational methods

Metastable allotropes of silicon recovered from high-pressure conditions exhibit a wide range of crystal structures, physical properties and transformation pathways that remain only partially understood despite decades of study. This article combines original crystallographic observations with a critical review of phase transformations, nucleation mechanisms and crystal growth processes in elemental Si and Na-Si systems synthesized under high-pressure, high-temperature conditions. Using in situ diffraction data, structural characterization and computational approaches, we analyze how symmetry breaking, lattice instabilities and kinetic constraints govern the formation of dense polymorphs (Si-II, Si-III, Si-XI) and open-framework structures, including clathrate and channel phases. Particular attention is given to the role of large-volume synthesis and chemically assisted growth routes in controlling phase selection, defect formation and recoverability. The evolution of hexagonal polytypes, including nanostructured 6H silicon, is discussed in terms of stacking modifications driven by stress release and thermal treatment. By integrating crystallographic relations, thermodynamic considerations and growth kinetics, this work identifies phase-transformation mechanisms as the key factor linking structure, synthesis conditions and functional properties of silicon allotropes. The results provide a unified framework for understanding crystal growth at high pressure and offer guidance for the controlled synthesis of advanced silicon materials.