Scientists have discovered that the secret to creating exotic forms of silicon lies in a surprisingly elegant principle: density matching.

For years, researchers have explored methods to make unusual crystal structures of silicon that could improve solar panels and electronics. These exotic phases have remarkable properties – one form called r8 silicon could make solar cells far more efficient – but creating them has required crushing silicon under extreme pressures.

Now, an international team has found a much simpler route by starting with a different form of silicon altogether. Instead of regular crystalline silicon, they used amorphous silicon – a disordered, glassy version of the material. When compressed at room temperature under pressures roughly 25% lower than traditional methods, this jumbled silicon transforms directly into the desired r8 structure.

The breakthrough centers on what the researchers call "density matching" – a phenomenon where materials naturally organize themselves when their densities align perfectly. As the amorphous silicon compresses, its flexible, disordered structure gradually rearranges itself until it reaches the same density as the target r8 crystal phase.

At that critical density point, something remarkable happens. The compressed amorphous material forms what the team calls a "medium-density amorphous" state – essentially a perfectly matched template that guides the silicon atoms into the r8 crystal arrangement. It's like having building blocks that suddenly realize they can fit together in a specific pattern.

The discovery represents the culmination of more than a decade of research, combining sophisticated high-pressure experiments with advanced computer modelling to unlock the secrets of how disordered materials behave under extreme conditions. Using X-ray diffraction at the High-Pressure Collaborative Team (HPCAT) beamline at 16-ID at the Advanced Photon Source, a U.S. Department of Energy (DOE) user facility at DOE’s Argonne National Laboratory, and neutron diffraction at DOE’s Oak Ridge National Laboratory, the researchers measured this density-driven transformation happen in real time. Large facilities such as the APS provide necessary in situ probes capable of monitoring the structural evolution of metastable silicon at atomic scale. This capability led to the discovery of the ordered silicon from a disorder form through density matching. 

The key insight is that the disordered starting material has enough structural flexibility to "side-step" the rigid pathways that crystalline materials must follow. The starting amorphous material doesn't need to follow the usual complex pathway through various metallic phases. Instead, the density match creates a direct shortcut to the desired structure. The team demonstrated this density-matching principle works for both silicon and germanium, suggesting it might be a universal pathway for creating exotic materials. 

The discovery has significant implications for many applications such as materials for energy generation. Exotic silicon phases like r8-Si could improve photovoltaic efficiency in solar cells by absorbing more of the solar spectrum, potentially making solar energy more cost-effective and accessible. Beyond solar applications, the density-matching approach could enable new materials for extreme environments and quantum sensing. – Jodie Bradby, Bianca Haberl and Guoyin Shen

See: B. Haberl¹, M. Guthrie¹, G.S. Jung², L.B.B. Aji³, J.J. Molaison¹, G. Shen⁴, S. Irle², J.E. Bradby⁵, “Pressure-driven density match nucleates metastable r8 phases from amorphous Si and Ge,” Mat Today 89, 140-149 (2025)

Author affiliations: ¹Neutron Scattering Division, Oak Ridge National Laboratory; ²AComputational Sciences and Engineering Division, Oak Ridge National Laboratory; ³Lawrence Livermore National Laboratory; ⁴HP-CAT, Argonne National Laboratory; ⁵The Australian National University.

The authors gratefully thank and acknowledge J.S.Williams (ANU) for many fruitful discussions, S.V. Sinogeikin (HPCAT, APS, now DACTools) for his assistance during X-ray data collection, S. Tkachev (GSECARS, APS) for the gas loading of the symmetric X-ray diamond cell, and R. Boehler (ORNL and HP-TECH) for the diamond anvil preparation for the neutron diamond cell. Portions of this work were funded by Laboratory Directed Research and Development (LDRD) funding, Oak Ridge National Laboratory, United States, and by the Australian Research Council (ARC) under grant number DP230100231. This research used resources at the Spallation Neutron Source, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory. This research also used resources from the Compute and Data Environment for Science (CADES) at the Oak Ridge National Laboratory, which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC05-00OR22725. The X-ray diffraction was performed at HPCAT (Sector 16), Advanced Photon Source (APS), Argonne National Laboratory. HPCAT operations are supported by DOE-NNSA’s Office of Experimental Sciences. The gas loading was performed at GeoSoilEnviroCARS (The University of Chicago, Sector 13), Advanced Photon Source (APS), Argonne National Laboratory. GeoSoilEnviroCARS was supported by the National Science Foundation, Earth Sciences (EAR–1634415). The Advanced Photon Source is a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.

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