Scientists find cheaper, scalable way to make rare R8 silicon

Oak Ridge National Laboratory and its partners have cut the pressure needed to make rare R8 silicon by about 25 percent, using amorphous silicon at room temperature to reach the phase at roughly 8 gigapascals. The advance matters because R8 is a nonnatural silicon form with strong potential for electronics and energy-storage applications, and the new route looks far more practical than the brute-force methods that came before.

The work, published in Materials Today, paired neutron diffraction at the Spallation Neutron Source in Oak Ridge, Tennessee, with X-ray diffraction at the Advanced Photon Source in Argonne, Illinois, then folded those measurements into high-resolution digital modeling. The result was a real-time view of how the material changed under pressure and a computational map of why the transformation worked. ORNL said the project took more than 10 years.

The key idea was density matching. Instead of starting from crystalline silicon and forcing it through much harsher conditions, the team began with amorphous silicon, a disordered and easier-to-make form of the element, then compressed it into a medium-density amorphous state. Under that load, the jumbled structure reorganized into crystalline R8. The paper says the metastable rhombohedral r8 phase nucleated from pure amorphous silicon and germanium during room-temperature compression below 10 GPa, while ORNL’s neutron highlight pins direct crystallization of R8 from amorphous silicon at about 8 GPa.

The research team included Bianca Haberl, Malcolm Guthrie, Gang Seob Jung, Leonardus B. Bayu Aji, Jamie Molaison, Guoyin Shen, Stephan Irle and Jodie Bradby, with broader participation from Lawrence Livermore National Laboratory and the Australian National University. Simulations on ORNL’s Compute and Data Environment for Science cluster helped the group test alternative transformation pathways and explain why the lower-pressure route works. Irle said the effort could deliver “tremendous energy savings” and be “highly scalable for industry.”

For the nuclear community, the most immediate payoff is not a reactor component tomorrow but a better route for probing and making a semiconducting phase that may matter in harsh environments. APS says R8 silicon could improve photovoltaic efficiency by absorbing more of the solar spectrum, and it also notes that the same density-matching approach may apply to carbon. The headline advance is the pressure cut, but the real shift is that R8 silicon now looks less like a lab curiosity and more like a manufacturable material path.