Silicon (Si) is ubiquitous in photovoltaic (PV) solar devices due to its abundance, low cost, and non-toxicity. Despite these advantages, the optical absorption of Si leaves much to be desired. This has generated increased interest in exotic phases of Si that retain the advantages of Si while having increased optical properties. One such phase is the rhombohedral (r8-Si) phase. Modelling studies suggest r8-Si has a significantly increased optical absorption, particularly across the solar spectrum, when compared to the types of Si currently used for PV applications.
This phase is formed via pressure application and is known to be stable between 2 - 9 GPa under conventional diamond anvil cell (DAC) compression. Unfortunately, r8-Si is not stable at ambient conditions and no significant amounts of r8-Si have been observed after DAC decompression. However, under point-loading a mixed structure comprised of r8-Si alongside a secondary phase (bc8-Si) can be recovered.
In our work, we investigate the crystal structure of the bc8/r8 mixed structure using x-ray diffraction (XRD). The relative volume of these phases was determined, with r8-Si being the dominant phase comprising around 70% of the mixed structure. Further, a distortion within the unit cell was observed along the direction of point-loading. Theoretical calculations together with these observations suggest the recovered material contains an intrinsic compression of around 4 GPa that stabilizes the r8-Si.