Advanced Synthesis of Na 4 Si 24
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Advanced Synthesis of Na4Si24 Michael Guerette1*, Timothy A. Strobel1, Haidong Zhang1, Stephen Juhl2, Nasim Alem2, Konstantin Lokshin3, Lakshmi Krishna4, P. Craig Taylor4 1
Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015 USA
2
Department of Chemistry, Penn State, University Park, PA 16802 USA
3
Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA
4
Department of Physics, Colorado School of Mines, Golden, CO 80401 USA
*Corresponding author, e-mail: [email protected]
ABSTRACT
The recently discovered orthorhombic allotrope of silicon, Si24, is an exciting prospective material for the future of solar energy due to a quasi-direct bandgap near 1.3 eV, coupled with the abundance and environmental stability of silicon. Synthesized via precursor Na4Si24 at high temperature and pressure (~850 °C, 9 GPa), typical synthesis results have yielded polycrystalline samples with crystallites on the order of 20 µm. Several approaches to increase the crystal size have yielded success, including in-situ thermal spikes and refined selection of the starting materials. Microstructural analysis suggests that coherency exists between diamond silicon (d-Si) and Na4Si24. This hypothesis has led to the successful attempts at single crystal synthesis by selecting large crystals of d-Si along with metallic Na as the precursors rather than powdered and mixed precursor material. The new synthesis approach has yielded single crystals of Na4Si24 greater than 100 µm. These results represent a breakthrough in synthesis that enables further characterization and utility. The promise of Si24 for the future of solar energy generation and efficient electronics is strengthened through these advances in synthesis.
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INTRODUCTION Na4Si24 is the precursor to Si24, a recently discovered allotrope of silicon with relevance to next-generation solar energy generation and efficient micro-electronics. With a quasi-direct bandgap near 1.3 eV [1], Si24 is well-suited to efficiently convert much of the solar spectrum [2] to electrical energy as both an intrinsic or doped material [3]. In the reverse process, this phase also holds promise as an efficient emitter of light for improved LED performance. In the quest for miniaturization, Si24 could allow more transistors on a chip due to the efficient hole-electron recombination. The fact that these properties can be achieved in silicon means that the industry infrastructure exists to utilize it, and it is environmentally stable/ non-toxic. Both Na4Si24 and Si24 are orthorhombic (Cmcm). Na4Si24 possesses the Eu4Ga8Ge16 type structure [4] also found in known alkaline earth silicides, AESi6 (AE = Ca, Sr, Ba) [5–7] and EuSi6 [8]. This compound consists of a channel-like sp3 silicon host structure filled with linear Na chains as a guest structure. It sh
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