Researchers resolve decades-old debate over compressed silicon in unprecedented detail

Researchers resolve decades-old debate over compressed silicon in unprecedented detail

When researchers shock pressure samples, they essentially compress the material in one direction. In many materials, this typically results in an irreversible deformation driven by the generation and propagation of defects, which are small defects in the atomic arrangement of the material. The researchers found that instead of accumulating defects, silicon prefers to relax through the collective movement of its atoms and transform into a high-pressure structure. Credit: Sylvia Pandolfi/Greg Stewart/SLAC . National Accelerator Laboratory

Silicon, an abundant element in the Earth’s crust, is the most widely used semiconductor material and is important in fields such as engineering, geophysics and plasma physics. But despite decades of studies, how matter transforms when exposed to powerful shock waves has been the subject of long-running debate.

“One might assume that because we have already studied silicon “In many ways, there’s nothing left to discover,” said Silvia Pandolfi, a researcher at the Department of Energy’s SLAC National Accelerator Laboratory. “But there are still some important aspects of its behavior that aren’t clear.”

Now, researchers at SLAC have finally put that controversy to rest, providing the first direct, high-resolution demonstration of how a single silicon crystal deforms during shock compression on nanosecond time scales. To do this, they studied the crystal with X-rays from the Linac Coherent Light Source (LCLS) X-ray laser. The team published their findings in Nature Connections On September 21. What they learn could lead to more accurate models that better predict what will happen to specific materials in harsh conditions.

“This is a great example of an experiment that is necessary to better understand certain materials,” said SLAC scientist Arianna Gleeson, who was the principal investigator. “You have to start simple, single crystals, to know what you’re tracking and understand in really detailed ways before you can build in complexity to make way, say, for the next semiconductor in the 21st century that will allow electronics industry To continue the remarkable progress we have seen in the past 50 years.”

time to relax

When researchers shock pressure samples, they essentially compress the material in one direction. This creates so much pressure in this single direction that the material needs to find a way to relax. In many materials, this usually leads to plasticity, which is an irreversible deformation caused by the generation and propagation of defects, which are small defects in the atomic arrangement of the material.

In previous studies, researchers interpreted various features seen in shock-compressed silicone as a sign of this plastic deformation. But molecular simulations suggested a different, flawless deformation.

“In our experiment, we showed that in the case of silicon, this conventional plasticity is not the main relaxation mechanism,” said Pandolfi, who led the analysis. “Instead of accumulating defects, silicon prefers to relax through the collective motion of its atoms and transform into a high-pressure structure. This has been predicted by calculation but so far has been very difficult to prove experimentally.”

يحل الباحثون جدلًا دام عقودًا حول السيليكون المضغوط بتفاصيل غير مسبوقة

Schematic view of our experimental composition and deformation mechanism. The computed XRD pattern fits our experimental data (upper right panel). The position of the single-crystal diffraction spots is calculated using the proposed orientation relationship between Si-I and Si-V, while the circles and lines serve as an eye guide and reference for the Debye-Scherrer rings. attributed to him: Nature Connections (2022). DOI: 10.1038 / s41467-022-33220-0

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In the The issue is in extreme conditions (MEC) At LCLS, researchers first sent a shock wave through the silicon sample using a carefully tuned optical laser, allowing them to reach extremely high temperatures and pressures. Next, they hit the sample with ultrafast X-ray laser pulses from the LCLS. Some of the X-rays are then scattered into a detector, allowing researchers to observe how the sample’s atoms rearrange in response to increasing pressure and temperature as the shock wave propagates over the ultrafast and ultramicroscales.

Previous studies focused on the collective response in samples consisting of many small crystals in different directions. This allowed the researchers to determine the mean behavior of the material but did not allow them to get the full picture. Working with a single crystal sample allowed the researchers to follow how the crystal transformed at the atomic level.

“The success of this experiment is due to the amazing combination of sample quality and X-rays from LCLS,” Pandolfi said. “We were able to get to basic materials science without having to deal with distortions from an X-ray source or minute structural defects in the sample that could change the behavior of the material. It’s much more than comparing ‘apples to apples’ with theory: it allowed us to visualize What happens in what is as close as possible to a typical system.”

Springboard Technology

The ability to gain insight into how materials respond to stress is the source of information for designing the next generation of X-ray facilities. Upgrading the material under extreme conditions (MEC-U) will produce unprecedented pressures accompanied by high-energy X-rays to explore a wider range of conditions, enabling the study of complex systems with direct relevance to future technologies. Examples range from the semiconductor industry to fusion energy and satellite protection.

“We’re looking at this as an evolution of a staging technology,” Gleeson said. “They can have many different applications beyond silicon, which really pushes the boundaries of new materials and helps us shed light on the path toward engineering these materials for the benefit of humanity.”

Get the physics right

To pursue this study, the researchers plan to try the technology on different samples with more complex compositions to better understand basic materials relevant to industry as well as Earth and planetary sciences.

“This new data really shows where the plasticity models used to predict how materials will behave makes the physics right and where we need to make changes,” Gleeson said. “There are other materials that respond in similar ways that are often used for industrial purposes. For example, high-performance coatings to withstand extreme conditions such as impacts of micro-meteorites on spacecraft. Understanding how these materials degrade over time in this environment will help us figure out how to prevent this in the future. It all starts with having accurate models at the atomic level.”

The researchers also plan to conduct more silicon experiments to see what happens when they compress the sample in different directions and see if where the stress is applied changes how the sample deforms. For Pandolfi, the possibility of revealing more unknowns about this pervasive and useful element is one of the most exciting aspects.

“I have such a soft spot for silicon,” Pandolfi said. “Anything new I can find out about it is very interesting to me.” “Sometimes I have to stop and think about what we’re talking about: we’re looking at the motion of atoms at high pressure over a billionth of a second. The fact that we can get such clear information about the phenomenon of people who have been investigating this for so long is amazing.”

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more information:
Sylvia Pandolfi et al., Mechanism of atomic deformation of silicon under laser-driven shock pressure, Nature Connections (2022). DOI: 10.1038 / s41467-022-33220-0

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