Three ways we’re exploring extreme materials at the Earth and Planets Laboratory

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Monday, October 18, 2021 

The center of our planet is a very different world than the one we live in at the surface. Temperatures and pressures are unfathomably large—the inner core is about 3 million times the atmospheric pressure at sea level.  When materials experience extreme pressures and temperatures, the very atomic structures that define them compress and reshape, causing them to change in sometimes surprising ways.

Carbon is a good example. Both graphite and diamond are made from carbon. Graphite, which forms at low pressure near the surface of the planet, is soft and opaque. You can smear it onto paper to write with it. On the other hand, diamond, which forms at high pressure in the deep mantle, is transparent and one of the hardest materials found on Earth.

The different arrangements of carbon atoms determine their atomic structures and physical properties, which affects how we engage with and use them every day. 

At the Earth and Planets Laboratory, we put materials through their paces to observe in the lab how their atoms shift and rearrange under the extreme temperatures and pressures they would face inside of a planet—sometimes creating entirely new materials in the process. 

Below we will explore exactly how we are studying extreme materials on campus. But first, a little history.

Carnegie’s high-pressure history

Until the 20th century, we could only imagine how materials would act under the extreme pressures and temperatures found in the deep interior of our planet.

The seminal work of Nobel Laureate Percy Bridgman, who pioneered high-pressure research in the early decades of the 20th century, broke this barrier with the invention of a piston-cylinder press. This invention allowed scientists to create and maintain pressures in the lab at around 2-3 kilobars—enough to study minerals under pressure up to about 80 kilometers into Earth’s crust and shallowest mantle. However, only a very few labs had access to such equipment.

F. R. Boyd with high pressure equipment
Geophysical Laboratory scientist Joe Boyd examines a sample in the Boyd-England press, which he developed with engineer Joseph England. This machine was the first to be able to consistently apply 40 kilobars of pressure—what materials experience in the deep crust and uppermost mantle. The press is still in use today.

In the early 1960s, the late Joe Boyd, a staff scientist at the Geophysical Laboratory (GL)* teamed up with Joseph England, the GL engineer, to create the Boyd-England Piston Cylinder. This press could reach and maintain pressures around 40 kilobars or more at temperatures exceeding 1600 degrees Celsius—finally allowing scientists to study the uppermost mantle. The machine was easy to build and use and remains a workhouse in high-pressure studies to this day. Still, the pressures and temperatures that would unlock the secrets of the rest of the mantle and core—most of the planet—were inaccessible. 

*The Geophysical Lab merged with the Department of Terrestrial Magnetism in 2020 to form the Earth and Planets Laboratory (EPL). 

In 1975, GL's Peter M. Bell and Ho-kwang Mao took the materials science world by storm. Following the development of the first diamond anvil cell by scientists at the National Bureau of Standards, which could apply around 500 kilobars between the tips of diamonds, Bell and Mao published a paper describing a new diamond anvil cell design that could apply pressures of 1 Megabar—double the previous record. About a decade later the same team was able to achieve pressures of 2.8 Megabars—again blowing the old record out of the water. 

Reaching the 2.8 Megabar threshold kicked off a new era of ultra-high-pressure science and opened the door to exploring material properties in the untapped pressure region above 1 Megabar. This innovation allowed access to pressures equivalent to Earth’s outer core for the first time. 

Studies exploring these extreme conditions provide a rare window into planetary interiors. At the same time, these pressures allow scientists to create entirely new technologically relevant materials with a whole host of novel physical phenomena. We do both at the Earth and Planets Laboratory. 

With each new pressure frontier, we unveil new details of our planet and of materials themselves. Today, the Earth and Planets Laboratory continues to be one of the premier institutions in the world engaged in high-pressure science. Our high-pressure work falls into three main categories:

Diamond Anvil Cell 1975
The diamond-anvil high-pressure cell, shown here, was perfected by Carnegie’s Ho-Kwang (Dave) Mao and his colleague Peter Bell in 1975 allowed scientists to reach 1 Mega. Opposed diamond anvils form the heart of the high-pressure cell.

Learn more about the history of the Geophysical Lab

​What is the fundamental behavior of materials under extreme conditions?

Under extreme conditions of pressure and temperature, the fundamental structures and properties of materials change, often to exotic states not found under normal conditions. Scientists at Carnegie are trying to understand the fundamental rules that dictate chemical bonding, phase transformations, and physical properties under these extreme conditions, often coupled with high strain rates and short time scales.

Here we see hydrogen crystallized into a solid at pressures 59,000 times atmospheric pressure. Image courtesy Carnegie Institution for Science and Timothy Strobel.

Generally speaking, materials under high pressures shrink in volume because the average space between atoms gets smaller. This new close-knit configuration allows different types of bonds to form between atoms, which can fundamentally change the properties of a material—like its electrical conductivity, thermal conductivity, viscosity, melting, and reaction kinetics.

What is a phase change?

 Under ambient conditions, silicon and carbon atoms are arranged in a diamond-like crystal structure (left image). Above 1 million atmospheres (100 GPa), silicon carbide (SiC) is transformed to a rock-salt structure, in which the atoms are squeezed much closer together (right). These are two different phases of silicon carbide. Image courtesy Carnegie Institution for Science/Sally June Tracy. 

Pressure generally changes a material’s properties on a smooth continuum—until it doesn’t!

At specific pressures, some materials’ properties change dramatically. These reproducible material boundaries designate phase changes in which the entire structure of the material suddenly and reliably rearranges. For example, molecular solids, like water ice, go through several phases of structural transition as pressure increases. Carnegie’s Timothy Strobel showed some of those phases during a live demonstration in a recent Neighborhood Lecture event.

In compounds that form rock (e.g., oxides and silicates), a central positively charged atom of metal like magnesium or silicon is surrounded by a bunch of negatively charged oxygens. As pressure increases, the number of oxygens that can fit around the central metallic cation increases, leading to a more tightly packed material. For example, in common quartz that we find at the surface (SiO2), the silicon is bound to four oxygens. However, under 80 kilobars of pressure, it transforms to a dense phase, called stishovite—a high-pressure version of SiO2 found as microscopic grains in impact craters and high-pressure rock. In stishovite, each silicon atom bonds to the nearest six oxygens surrounding it instead of four, but both quartz and stishovite are different phases of silicon dioxide.

Quartz versus Stishovite.png
In common quartz we find at the surface (SiO2), silicon is bound to four oxygens. However, under pressure, it transforms to a dense phase, called stishovite, with each silicon atom bonding to the nearest six oxygens surrounding it—both are different versions of silicon dioxide. Image courtesy of Wikipedia user Materialscientist. 

Scientists like those at EPL investigate the changing properties of materials under pressure. They are constantly seeking to expand their understanding of materials to apply that knowledge to models of planetary interiors and seek out new materials with properties that can help us expand our current technologies.


Elucidating how asymmetry confers chemical properties

You’ve heard the expression, form follows function? In materials science, function follows form.

New research by Carnegie’s Olivier Gagné and collaborator Frank Hawthorne of the University of Manitoba categorizes the causes of structural asymmetry, some surprising, which underpin useful properties of crystals, including ferroelectricity, photoluminescence, and photovoltaic ability.

Conductivity under pressure

Anne Pommier’s investigation into the conductivity of minerals under pressure has several practical uses. When scientists measure electromagnetic activity in the field, they see “electrical anomalies” where the conductivity is higher or lower than the surrounding material. Magmas and aqueous fluids tend to stand out as huge, highly conductive anomalies—making them a clear target for conductivity studies.

Pommier’s work to understand melts and minerals’ electrical conductivity under different pressures and temperatures will allow scientists to compare these electromagnetic field measurements to lab collected conductivity data.

“We know the conductivity from the field measurements. But the big question is, ‘What is it?’ Under an active volcano, magma is expected—but what type of magma? At what temperature and depth is it stored? How much is there?” Pommier continued, “The electrical models we create in the lab will help identify the nature and storage conditions of magma in the crust and upper mantle.”

How do extreme materials affect planetary processes and properties?

Given the extreme conditions found within planetary interiors and involved in planetary formation processes, understanding the structures and properties of materials found in these states is critical to our understanding of Earth and other planets in our solar system and beyond. Carnegie scientists perform experiments to simulate deep planetary interiors and impact processes to determine their atomic structures and heat/radiation/electrical transport properties. Image courtesy Carnegie Institution for Science and Katy Cain. 

We cannot take direct samples from the deepest interior of our planet. The Earth’s core is about 2,900 km below the surface. Meanwhile, the furthest we’ve managed to drill is only about 12 km, which doesn’t even get us past the crust and extend into the mantle. Sometimes we can find natural samples of high-pressure materials at meteorite impact sites or trapped inside of a diamond, but normally the best we can do is take samples of the materials we think are inside our planet and test them in the lab.

Kelsey Prissel places her assembly into the multi-anvil press.
Former Postdoctoral Fellow Kelsey Prissel loads a mineral sample into a multi-anvil press. Image courtesy Carnegie Institution for Science and Katy Cain.

We perform this foundational research using high-pressure devices such as diamond anvil cells and large-volume presses such as the piston cylinder. We load precursor materials into these devices and then subject them to high pressures and temperatures to push the material to the limit. Scientists create detailed logs of how its mineralogy and properties change.

We can then use what we find to inform the physics and chemistry in computational models of planetary interiors—enhancing our understanding of how our planet developed and changed through time.

Probing the Earth’s core

Geodynamo and Sun
Studying material properties under intense temperatures and pressures can help us understand important planetary processes, like what is driving the magnetic field that protects our planet from solar winds.

For instance, this type of research can help us understand what is driving Earth’s geodynamo—the planetary engine of convecting metals that drives our planet’s protective magnetic field. We know that our planet came from the disk of dust and gas that surrounded our Sun in its youth. Eventually, the densest material sank inward in the forming planet, creating the layers that exist today—core, mantle, and crust. Although the core is predominantly iron, seismic data indicates that some lighter elements like oxygen, silicon, sulfur, carbon, and hydrogen, were dissolved into it as it developed.

Over time, the inner core crystallized and has been continuously cooling since then. Scientists want to know if heat flowing out of the core and into the mantle could drive the geodynamo on its own. Or does this thermal convection need an extra boost from the buoyancy of light elements, not just heat, moving out of a condensing inner core?

A team that included Carnegie’s Alexander Goncharov recently addressed this question by directly measuring the thermal conductivity of solid iron and iron-silicon alloys up to 1.14 megabars and 3300K—mimicking conditions found in Earth’s outer core. 

The team found that with a concentration of about eight weight percent silicon in their simulated inner core, the geodynamo could have functioned on heat transmission alone for the planet’s entire history.

A third extreme condition: time!

Extreme heat and pressure aren’t only found in the center of the planet! Sometimes they occur during sudden events—like when a meteor collides with a planet. When large impacts occur, time is also at an extreme. A material might go through extreme heat and pressure changes in less time than it takes to snap your fingers. Carnegie’s Sally June Tracy is an expert at studying materials under these conditions.

She uses gas guns or high-powered pulsed lasers to generate shock waves that travel through a sample producing a fleeting high pressure-temperature state. During the experiment, she collects data using X-ray or laser probes to learn how the material changes on a structural and chemical level.

Says Tracy, “This work provides experimental constraints on subjects ranging from planetary formation, high-pressure phases of deep planetary interiors, and materials for extreme applications.”


New form of quartz discovered

When a meteorite hurtles through the atmosphere and crashes to Earth, how does its violent impact alter the minerals found at the landing site? What can the short-lived chemical phases created by these extreme impacts teach scientists about the minerals existing at the high-temperature and pressure conditions found deep inside the planet?

New work led by Carnegie’s Sally June Tracy examined the crystal structure of the silica mineral quartz under shock compression and is challenging longstanding assumptions about how this ubiquitous material behaves under such intense conditions. The results are published in Science Advances.

When it comes to planetary habitability, its what’s inside that counts

Which of Earth’s features were essential for the origin and sustenance of life? And how do scientists identify those features on other worlds?

A team of Carnegie investigators with array of expertise ranging from geochemistry to planetary science to astronomy published in Science an essay urging the research community to recognize the vital importance of a planet’s interior dynamics in creating an environment that’s hospitable for life.

Examining exoplanet habitability from the inside out

On Earth, the interior dynamics and structure of the silicate mantle and metallic core drive plate tectonics, and generate the geodynamo that powers our magnetic field and shields us from dangerous ionizing particles and cosmic rays. Life as we know it would be impossible without this protection. Similarly, the interior dynamics and structure of super-Earths will shape the surface conditions of the planet.

With exciting discoveries of a diversity of rocky exoplanets in recent decades, are much-more-massive super-Earths capable of creating conditions that are hospitable for life to arise and thrive?

Knowledge of what’s occurring beneath a super-Earth’s surface is crucial for determining whether or not a distant world is capable of hosting life. But the extreme conditions of super-Earth-mass planetary interiors challenge researchers’ ability to probe the material properties of the minerals likely to exist there.

That’s where lab-based mimicry comes in.

How can we improve technological materials using extreme conditions?

Given the vast number of new structures and properties that appear under extreme conditions, many of these are applicable to novel technological development. For example, new superhard materials, efficient semiconductors, and high-temperature superconductors have all been discovered under extreme pressures. Carnegie scientists are pushing the boundaries to uncover these novel materials with exciting properties, and develop innovative strategies to recreate/stabilize them under normal conditions. Image courtesy of Carnegie Institution for Science and Timothy Strobel

In a recent Neighborhood Lecture, someone in the audience asked Carnegie’s Tim Strobel what the top global challenges materials science can help solve.

“Clean water, renewable energy, world peace—you name it, and materials science can help solve it,” said Strobel. “Just look around where you’re sitting right now; maybe you’re at your desk looking at your computer screen. These are all materials with particular properties. The world we live in is possible because of materials science and novel materials research”.

In addition to allowing scientists to directly observe minerals behaving as they would in the center of the planet, EPL researchers like Strobel harness these high-pressure tools to create brand new materials that usually* only form under very specific high-temperature and pressure conditions. We now use computational chemistry to predict materials and then work in the lab to synthesize them and study their properties.

Things like next-generation superconductors, photovoltaics, and insulators are all on the table when pressure is involved.

*While many of these exotic materials are discovered under extreme conditions, scientists can develop novel techniques to recreate them under more normal conditions. For example, diamond, the high-pressure form of carbon, can also be synthesized under low-pressure conditions using vapor deposition techniques. Nature figured out how to do this already as nanometer diamonds are a common component of interstellar dust.

Searching for superconductors

Of specific interest to our campus (and the world at large) is the search for new superconducting materials. A superconductor has no electrical resistance; it does not restrict electron movement or lose any energy as electrons travel through it. A stable room temperature superconductor—the holy grail of materials science—would revolutionize technology by substantially reducing energy use and massively increasing the speed of computing.

Typically these materials have to be cooled below a very low temperature, which often makes them impractical for widespread use.

Discovering a superconductor that exists at ambient room temperatures and pressures is likely more than a decade off, but we are building the path that will get us there. 

At Carnegie, the search for the superconductor goes back to the 1980s. In 1987, Carnegie scientists Ho-kwang Mao and Robert Hazen, identified the composition and structure of the first high-temperature superconductor. (If you want to learn more about that, Hazen wrote a popular book The Breakthrough: The Race for the Superconductor.) We've been searching for superconducting materials ever since! 

More recently, Strobel and his team predicted and synthesized the first carbon-based clathrate. Clathrates are cage-like structures that can trap other elements inside. Scientists more than 50 years ago predicted that a carbon-based clathrate would have useful properties, like the potential for superconductivity. 

Former Carnegie postdoc Li Zhu said in an interview, “We used a structure prediction method, which we developed, to predict the first stable carbon clathrate. Then, my colleagues used that prediction to synthesize the clathrate in the lab under very high pressures. So, this is the first carbon clathrate in the world.”

He continued, “This material is as hard as diamond and has the potential to act as a superconductor or an insulator depending on which atoms we put in the cage.”

The team has since confirmed superconductivity in these carbon-boron clathrates, a paper laying out their discovery is in the works.


New form of silicon may enable next-gen electronic devices

A team led by Carnegie’s Thomas Shiell and Timothy Strobel developed a new method for synthesizing a novel crystalline form of silicon with a hexagonal structure that could potentially be used to create next-generation electronic and energy devices with enhanced properties that exceed those of the “normal” cubic form of silicon used today.

Every day, the Earth and Planets Laboratory builds on the Geophysical Lab’s legacy to push the boundaries of high-pressure science. We develop techniques, probe new experimental questions, expand our computation capabilities, collaborate with colleagues, and fill our labs with top-tier instrumentation (and scientists).

All of this allows us to make new observations about our planet’s interior, better understand planetary development, and ultimately develop new materials that could make our world a better place.