Abstract
The kinetics of two metastable polymorphs of silicon under thermal annealing was investi-
gated. These phases with body-centred cubic bc8 and rhombohedral r8 structures can be
formed upon pressure release from metallic silicon. In the present study, these metastable
polymorphs were formed by two di erent methods, via point loading and in a diamond-anvil
cell. Upon thermal annealing di erent transition pathways were detected. In the point
loading case, the previously reported Si-XIII formed and was con rmed as a new phase
with an as yet unidenti ed structure. In the diamond-anvil cell case, bc8-Si transformed
to the hexagonal-diamond structure at elevated pressure, consistent with previous studies
at ambient pressure. In contrast, r8-Si transformed directly to diamond-cubic Si at a
temperature of 255C. These data were used to construct diagrams of the metastability
regimes of the polymorphs formed in a diamond anvil cell and may prove useful for potential
technological applications of these metastable polymorphs.
