Synthesis of Bulk BC8 Silicon Allotrope by Direct Transformation and Reduced-Pressure Chemical Pathways

Abstract

Phase-pure samples of a metastable allotrope of silicon, Si-III or BC8, were synthesized by direct elemental transformation at 14 GPa and ∼900 K and also at significantly reduced pressure in the Na-Si system at 9.5 GPa by quenching from high temperatures ∼1000 K. Pure sintered polycrystalline ingots with dimensions ranging from 0.5 to 2 mm can be easily recovered at ambient conditions. The chemical route also allowed us to decrease the synthetic pressures to as low as 7 GPa, while pressures required for direct phase transition in elemental silicon are significantly higher. In situ control of the synthetic protocol, using synchrotron radiation, allowed us to observe the underlying mechanism of chemical interactions and phase transformations in the Na-Si system. Detailed characterization of Si-III using X-ray diffraction, Raman spectroscopy, (29)Si NMR spectroscopy, and transmission electron microscopy are discussed. These large-volume syntheses at significantly reduced pressures extend the range of possible future bulk characterization methods and applications.

BC8 Silicon (Si-III) is a Narrow-Gap Semiconductor

“…Like previous studies, we confirm that LDA and GGA (PBE) both result in small direct overlap at the H point. After full relaxation within PBE, we obtain a = 6.6569 Å and x = 0.1014, in good agreement with our experimental values of a = 6.62767(3) Å and x = 0.10179(3) [14]. After applying the GW correction to the PBE result, direct band overlap of ~0.15 eV still exists at the H point, which is in agreement with previous theoretical calculations [21].…”

“…Slow decompression leads to the R8 phase between 10-2 GPa and further unloading to atmospheric pressure leads to BC8-Si, which persists at ambient pressure unless heated above ~200 o C [12,13]. While complete transformation of DC-Si to β-Sn-Si (and thereby BC8) requires pressures in excess of 10 GPa, recent studies indicate that chemical pathways involving sodium can reduce the formation pressure to as low as 7 GPa without the formation of β-Sn-Si [14]. In addition, an ambient-pressure colloidal synthesis route for BC8-Si nanoparticles was recently reported [15], which may serve useful for solar energy conversion applications with multiple exciton generation [16,17].…”

“…Detailed characterization of samples can be found in ref. [14]. An alternative set of phase-pure samples was obtained at 9.5 GPa in the Na-Si system [14].…”

“…[14]. An alternative set of phase-pure samples was obtained at 9.5 GPa in the Na-Si system [14]. These "chemical samples" were found to exhibit very similar physical properties, and are therefore not discussed separately.…”

High pressure: a feasible tool for the synthesis of unprecedented inorganic compounds

“…With DAC, phase-pure samples of a metastable allotrope of silicon, Si-III or BC8, were synthesized by direct elemental transformation at 14 GPa and about 900 K and also at significantly reduced pressure in the Na-Si system at 9.5 GPa by quenching from approximately 1000 K, and this could help us to tackle challenges in electronic and photovoltaic applications. 167 Because of their remarkable mechanical properties, Mg-transition-metal (TM)rare-earth (RE) alloys containing a synchronized long-periodstacking order (LPSO) structure have received significant attention. Unexpectedly, under high pressure (5 GPa) and high temperature (723 K), a new type of long-period superlattice Mg 77 Zn 9 Yb 14 was found in Mg 97 ZnYb 2 alloys.…”