Advanced Synthesis of Na4Si24, the precursor to Si24

What is it about?

This work describes some synthesis advances which has yielded large single crystals of Na4Si24. This is the high pressure/ temperature precursor material from which Si24 is derived, a newly discovered allotrope of silicon with desirable optical/ electronic properties.

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Why is it important?

Na4Si24 is a difficult phase to grow, due to a very narrow range of thermodynamic stability (P~9 GPa, T~850c). Previous synthesis techniques yielded 5-20 micron grain size of this phase, and these newly reported techniques yield grain sizes greater than 100 microns. This newly achieved size scale is large enough to allow several techniques which allow intrinsic property determination, without effects of phase impurity or grain boundary effects. Not only are we now able to understand this phase better, the improved synthesis of this phase removes a significant hurdle of obtaining large single crystals of Si24. A quasi-direct bandgap near 1.3 eV makes Si24 an interesting material for next generation solar energy conversion.