Researchers using the HP-CAT 16-ID-B beamline at the APS have obtained important data about an entirely new form of silicon they synthesized — an orthorhombic allotrope of silicon represented as Si24 — by utilizing a novel, two-step thermal diffusion methodology, the high-pressure precursor process.
Silicon is ubiquitous in contemporary technology. The most stable form of silicon at ambient conditions takes on the structure of diamond (cF8, d-Si) and is an indirect bandgap semiconductor, which prevents it from being considered as a next-generation platform for semiconductor technologies. Here, we report the formation of a new orthorhombic allotrope of silicon, Si24, using a novel two-step synthesis methodology. First, a Na4Si24 precursor was synthesized at high pressure; second, sodium was removed from the precursor by a thermal ‘degassing’ process.
The Cmcm structure of Si24, which has 24 Si atoms per unit cell (oC24), contains open channels along the crystallographic a-axis that are formed from six- and eight-membered sp3 silicon rings. This new allotrope possesses a quasidirect bandgap near 1.3 eV. Our combined experimental/theoretical study expands the known allotropy for element fourteen and the unique high-pressure precursor synthesis methodology demonstrates the potential for new materials with desirable properties.
Duck Young Kim, Stevce Stefanoski, Oleksandr O. Kurakevych and Timothy A. Strobel, “Synthesis of an Open-framework Allotrope of Silicon,” Nature Materials 14,169–173 (2015), DOI:10.1038/nmat4140, Published Online November 17, 2014.