Abstract
The Group 14 element silicon possesses a complex free-energy landscape with many (local) minima,
allowing for the formation of a variety of unusual structures, some of which may be stabilized at
ambient conditions. Such exotic silicon allotropes represent a significant opportunity to address the
ever-increasing demand for novel materials with tailored functionality since these exotic forms are
expected to exhibit superlative properties including optimized band gaps for solar power conversion.
The application of pressure is a well-recognized and uniquely powerful method to access exotic
states of silicon since it promotes large changes to atomic bonding. Conventional high-pressure
syntheses, however, lack the capability to access many of these local minima and only four forms of
exotic silicon allotropes have been recovered over the last 50 years. However, more recently, signifi-
cant advances in high pressure methodologies and the use of novel precursor materials have yielded
at least three more recoverable exotic Si structures. This review aims to give an overview of these
innovative methods of high-pressure application and precursor selection and the recent discoveries of
new Si allotropes. The background context of the conventional pressure methods and multitude of
predicted new phases are also provided. This review also offers a perspective for possible access
to many further exotic functional allotropes not only of silicon but also of other materials, in a technologically
feasible manner
