Properties of "exotic" ST12-germanium

January 03, 2017

Germanium may not be a household name like silicon, its group-mate on the periodic table, but it has great potential for use in next-generation electronics and energy technology.

Of particular interest are forms of germanium that can be synthesized in the lab under extreme pressure conditions. However, one of the most-promising forms of germanium for practical applications, called ST12, has only been created in tiny sample sizes—too small to definitively confirm its properties.

"Attempts to experimentally or theoretically pin down ST12-germanium's characteristics produced extremely varied results, especially in terms of its electrical conductivity," said the Geophysical Laboratory's (GL) Zhisheng Zhao, the first author on a new paper about this form of germanium.

The study's research team, led by GL's Timothy Strobel, was able to create ST12-germanium in a large enough sample size to confirm its characteristics and useful properties. Their work is published by Nature Communications.

"This work will be of interest to a broad range of readers in the field of materials science, physics, chemistry, and engineering," explained GL's Haidong Zhang, the co-leading author.

ST12-germanium has a tetragonal structure—the name ST12 means "simple tetragonal with 12 atoms."(See illustration below.) It was created by putting germanium under about 138 times normal atmospheric pressure (14 gigapascals) and then decompressing it slowly at room temperature.

The millimeter-sized samples of ST12-germanium that the team created were large enough that they could be studied using a variety of spectroscopic techniques in order to confirm its long-debated characteristics.

Like the most common, diamond-cubic form of germanium, they found that ST12 is a semiconductor with a so-called indirect band gap. Metallic substances conduct electrical current easily, whereas insulating materials conduct no current at all. Semiconducting materials exhibit mid-range electrical conductivity. When semiconducting materials are subjected to an input of a specific energy, bound electrons can be moved to higher-energy, conducting states. The specific energy required to make this jump to the conducting state is defined as the "band gap." While direct band gap materials can effectively absorb and emit light, indirect band gap materials cannot.

"Our team was able to quantify ST12's optical band gap--where visible light energy can be absorbed by the material—as well as its electrical and thermal properties, which will help define its potential for practical applications," Strobel said. "Our findings indicate that due to the size of its band gap, ST12-germanium may be a better material for infrared detection and imaging technology than the diamond-cubic form of the element already being used for these purposes."

The other Carnegie members of the team were GL's Duck Young Kim, and Emma Bullock, as well as Wentao Hu of Yanshan University.

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Hydrogen-stuffed, quartz-like water ice

August 19, 2016

Check out our new paper in JACS.

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Bhadram wins best poster @GRS

August 25, 2016

Free Postdoc Venkat Bhadram was one of three selected for best poster awards at the Gordon Research Seminar, which was held before the Gordon Research Conference in July at the Holderness School in New Hampshire. The winners of the poster awards gave five-minute short work presentations during the Gordon Research Conference. His poster, "High-pressure synthesis and characterization of ultraincompressible titanium pernitride," was co-authored by former EFree Technical Coordinator Duck Young Kim and EFree Associate Director Tim Strobel.

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Titanium Pernitride

March 7, 2016

Scientists from Carnegie have discovered a new transition metal pernitride, TiN2, which is ultraincompressible (bulk modulus ~360-385 GPa), and potentially a superhard material. Using a laser-heated diamond anvil cell, titanium nitride (TiN) and nitrogen (N2) were compressed to 73 GPa and heated to 2400 K. At these extreme conditions titanium nitride reacted with nitrogen to oxidize the titanium and form a new compound, titanium pernitride (TiN2).

This discovery is a part of an ongoing search for new semiconductor materials in the Ti-O-N system that could possibly be useful for photocatalytic water splitting reactions to produce H2. TiO2 is a well-known semiconductor photocatalyst that works in the UV region of the solar spectrum due to its wide band gap (~3 eV). It is established that nitrogen doping within TiO2 reduces its band gap and enhances its photocatalytic properties in the visible light region. But how would these properties change if all of the oxygen in TiO2 is replaced with nitrogen? Although there have been a few theoretical reports that have predicted the crystal structure and properties of TiN2, there has been no experimental evidence to validate these predictions.

While high pressure and highvtemperature conditions were used for its synthesis, TiN2 is a dynamically stable at ambient conditions and exhibits a unique crystal structure containing single-bonded dinitrogen units (pernitride ions, N24-) and Ti4+. The appearance of single N–N bonds in materials is rare as it requires filling of high-energy antibonding molecular orbitals of dinitrogen. The N–N bond length in TiN2 is ~1.383 Å and is comparable to the F–F single bond length (1.42 Å) in the F2 molecule, which is isoelectronic to the pernitride ion. The filling of antibonding orbitals serves to elongate the N-N covalent bonds and makes the material more resistant to external stress. Several pernitride compounds were discovered previously, but with transition metals belonging to the noble metal group. TiN2 is the first non-noble metal pernitride and the lowest-density transition metal pernitride synthesized to date. This work, which is published in Chemistry of Materials, has important implications for the synthesis of new metal nitrides and the understanding of their structure–property relationships. TiN2 is metallic, but its synthesis and properties place important end-member constraints on new photocatalysts in the ternary Ti-O-N system.

You can find the article here.

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Super Silicon in "New Scientist"

November 18, 2015

The November issue of New Scientist talks about hacking silicon's structure to make it more efficient for use in computer chips and solar panels. The element may have a whole valley named after it, but the atomic structure limits its ability to conduct electricity. This means that while silicon is ubiquitous, it's efficiency eventually hits a wall. But research from our lab might lead to new and improved silicon!

You can find the article here.

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Li-stuffed, B-doped carbon clathrates

September 14, 2015

The experimental synthesis of carbon clathrate materials is an, as of yet, unaccomplished experimental feat. Inside the cages of these hypothetical compounds there is precious little room, even for the smallest of atoms. In this work we explore a possible solution: dope the framework with boron!

You can find the paper here.

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New Allotrope of Silicon

November 1, 2014

Silicon is the second most-abundant element in the earth's crust. When purified, it takes on a diamond structure, which is essential to modern electronic devices--carbon is to biology as silicon is to technology. A team of Carnegie scientists led by Timothy Strobel has synthesized an entirely new form of silicon, one that promises even greater future applications. Their work is published in Nature Materials. Although silicon is incredibly common in today's technology, its so-called indirect band gap semiconducting properties prevent it from being considered for next-generation, high-efficiency applications such as light-emitting diodes, higher-performance transistors and certain photovoltaic devices.

Metallic substances conduct electrical current easily, whereas insulating (non-metallic) materials conduct no current at all. Semiconducting materials exhibit mid-range electrical conductivity. When semiconducting materials are subjected to an input of a specific energy, bound electrons can move to higher-energy, conducting states. The specific energy required to make this jump to the conducting state is defined as the "band gap." While direct band gap materials can effectively absorb and emit light, indirect band gap materials, like diamond-structured silicon, cannot.

In order for silicon to be more attractive for use in new technology, its indirect band gap needs to be altered. Strobel and his team--Duck Young Kim, Stevce Stefanoski and Oleksandr Kurakevych (now at Sorbonne)--were able to synthesize a new form of silicon with a quasi-direct band gap that falls within the desired range for solar absorption, something that has never before been achieved. The silicon they created is a so-called allotrope, which means a different physical form of the same element, in the same way that diamonds and graphite are both forms of carbon. Unlike the conventional diamond structure, this new allotrope consists of an interesting open framework, called a zeolite-type structure, which is comprised of channels with five-, six-and eight-membered silicon rings. They created it using a novel high-pressure precursor process. First, a compound of silicon and sodium, Na4Si24, was formed under high-pressure conditions. Next, this compound was recovered to ambient pressure, and the sodium was completely removed by heating under vacuum. The resulting pure silicon allotrope, Si24, has the ideal band gap for solar energy conversion technology, and can absorb, and potentially emit, light far more effectively than conventional diamond-structured silicon. Si24 is stable at ambient pressure to at least 842 degrees Fahrenheit (450 degrees Celsius).

"High-pressure precursor synthesis represents an entirely new frontier in novel energy materials," remarked Strobel. "Using the unique tool of high pressure, we can access novel structures with real potential to solve standing materials challenges. Here we demonstrate previously unknown properties for silicon, but our methodology is readily extendible to entirely different classes of materials. These new structures remain stable at atmospheric pressure, so larger-volume scaling strategies may be entirely possible."

"This is an excellent example of experimental and theoretical collaboration," said Kim. "Advanced electronic structure theory and experiment have converged to deliver a real material with exciting prospects. We believe that high-pressure research can be used to address current energy challenges, and we are now extending this work to different materials with equally exciting properties."

You can find the paper here.

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Check out these images and links.


Khattar Selected as 2014 Siemens Competition Semifinalist

October 22, 2014

The Geophysical Laboratory's Saelig Khattar was named a Seminfinalist for the 2014 Siemens Competition in Math, Science & Technology. The Siemens Foundation announced the Semifinalists this week on 22 October 2014.

Thanks to the superior quality of work submitted by students like Khattar, and his partner Ki Wan Kim, the Siemens Competition is the most challenging and prestigious research-based high school science contest in the country. Being selected as a Semifinalist is an exceptional achievement. Out of nearly 4,500 students who submitted projects to the Siemens Competition, only 408 students were named Semifinals.

The Siemens Competition in Math, Science & Technology fosters intensive research that improves students' understanding of the value of scientific study and informs their consideration of future careers in these disciplines.

This year's finalists are listed on the Siemens Foundation website.

Saelig Khattar is an intern at the Geophysical Laboratory from Thomas Jefferson School for Science and Technology. He is working with Tim Strobel on a novel method of synthesizing methanol through non-catalytic high-pressure CO2 hydrogenation reactions. His other research interests include material sciences, quantum physics, chemistry and nanotechnology.

Congratulations, Saelig!

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The Big Squeeze

December 16, 2013

High-pressure research featured in the NY Times

Check it out here

Supercharged Carbon

September 10, 2013

A research team from the Geophysical Laboratory, including Oleksandr Kurakevych, Timothy Strobel, Duck Young Kim and George Cody, has reported the synthesis of an ionic semiconductor, Mg2C, under high-pressure, high-temperature conditions, which is fully recoverable to ambient conditions.

In Mg2C, carbon becomes an anion with an extremely negative charge of 4e-. Purely ionic sp3 carbon in Mg-C carbides, namely C4- anions in Mg2C, was theoretically suggested by Marvin Cohen and colleagues at UC Berkeley in 1993, however, its experimental realization has not been reported until now.

The research team has a strong momentum to find novel metal-carbon compounds under extreme conditions. Motivated by their theoretical calculations predicting Mg2C formation above 15 GPa, they successfully synthesized the high-symmetry structure. This ion has sp3 bonding nature and thus, this finding completes a trilogy of sp1/sp2/sp3 bondings known in Mg-C compounds.

Unlike previously reported Be2C, Mg2C is a fully ionic semiconductor, producing methane gas (CH4) when hydrolyzed by the moisture in air. In the accompanying illustration, each carbon atom (grey colored balls) is located at fcc sites and magnesium atoms (green color balls) sit at the tetrahedral sites. The isosurface of charge density (metallic skin) is in a spherical shape, indicating the bonding nature could be assigned to be ionic or covalent. The color map on the top slice of the Mg2C unit cell shows there are no localized electrons between carbon atoms and confirms the ionic nature of the bonding.

"It is quite common to find unusual compounds under extreme conditions, especially in metal-carbon systems. We may even expect to stabilize high-pressure phases at ambient conditions, as is the case for Mg2C," said Kim.

"The recoverable nature of the sp3 C4- anion may open entirely new possibilities for the solid-state synthesis of new advanced materials," remarked Strobel.

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Several New Faces

September 10, 2013

Several new faces have appeared around the lab. A warm welcome to Manuvesh, Zhisheng, Yangzheng and Huiyang! Check out the People section to learn more.

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    Manuvesh Sangwan          Zhisheng Zhao            Yangzheng Lin            Huiyang Gou


Stevce Stefanoski joins the lab

March 10, 2013

Stevce Stefanoski joins the Geophysical Laboratory this week from the University of South Florida as a new postdoctoral associate. Stevce will be working on synthesis and physical characterization of carbon materials.

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New paper on silicon clathrates Published in Crystal Growth & Design

January 15, 2013

Silicon is the heart of modern electronics, in both pure element and compound forms. It is also one of the few elements capable of forming chemical analogs of gas hydrates, or clathrates—structures that form cages or networks that trap appropriately sized guest atoms.

Tim Strobel, a Geophysical Laboratory staff scientist and the team leader, works on different types of clathrates, both in molecular and covalent systems. Together with Oleksandr Kurakevych, Duck-Young Kim, Takaki Muramatsu, and Viktor Struzhkin, they study synthesis and thermodynamic stability of silicon and carbon analogs of water clathrates.

By continuously increasing the pressure applied to a mixture of elemental Na and Si, the crystallization product changes from elemental Si to NaSi6. At a high enough pressure, the high-symmetry diamond structure loses its stability and passes into low-symmetry tunnel structure, in which the three dimensions of space become non-equivalent. Despite high-pressure, high-temperature formation conditions, the researchers found that this new compound is completely recoverable to ambient conditions.

The team discovered that all sodium-silicon clathrate phases are only stable at high-pressure conditions. This is crucial for obtaining the crystals with exceptional electrical properties, and is unachievable by conventional chemical synthesis routes. "Synthesis under equilibrium conditions is the easiest and most efficient way to achieve the best material's performance," Kurakevych said. "High pressure synthesis remains the method of choice for characteristics improvement of high-pressure phases: previously for mechanical and optical properties of diamond and cubic boron nitride, now for electrical properties of silicon clathrates."

Another fascinating aspect of this work relates to making high-pressure materials that exhibit superlative properties, without using pressure. "Because these materials are only truly thermodynamically stable under high-pressure conditions, all previous synthesis reports of silicon clathrates may be viewed as the metastable synthesis of high-pressure phases at low-pressure conditions via a chemical precursor route. These findings identify silicon clathrates as the first example of this type of synthesis approach, and suggest that the synthesis of other high-pressure structures is possible by starting from the appropriate chemical precursor," said Strobel.

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This study can be found at: O.O. Kurakevych et al., Cryst. Growth & Design ,13, 303 (2013).


Tungsten takes on hydrogen

April 1, 2012

The low solubility of hydrogen in tungsten has led to its use as a gasket material to seal hydrogen in diamond anvil cell experiments at high pressures. This story, reported in J. Phys.: Condens. Matter, begins with a finding by Tim Strobel of the Carnegie Institution of Washington (CIW) that the gasket is not inert. He noticed that a tungsten hydride of WH composition can be formed under pressure. A similar finding was reported by a group at the University of Hyogo earlier.

The Cornell theory group, consisting of both chemists and physicists, and with a history of collaboration with the CIW experimentalists, was intrigued. They asked: Can other WHn phases be stabilized under pressure? And what might they look like?

An exploration of the rich structural landscape of tungsten hydrides ensued. Four tungsten-hydrogen compositions—WH, WH2, WH4 and WH6—appear stable, each in its own pressure range. The structures of the predicted extended hydrides are fascinating. They begin with an anti-NiAs-type structure for WH (in agreement with experiment)—a trigonal prism of hydrogens around each tungsten. With an increasing number of Hs in the chemical formula, the number of hydrogens around each tungsten atom increases, while the tungsten atoms remain about the same distance apart.

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The theoretical findings led the CIW group to direct efforts towards detecting the higher hydrides. Using excess hydrogen, different forms of tungsten, and heating, resulted in a better characterization of WH (see figure below), but no sign of the predicted higher hydrides. Attempts have been made to explain the absence of the higher hydrides through kinetic and entropy considerations, but essentially a mystery remains, an apparent disagreement between theory and experiment. The story will not end here, and there is at least a caution here for experimentalists: Whatever you use for the gasket, chemistry has a way of rearing its head, especially at high pressure.

This study can be found at: P. Zaleski-Ejgierd et al., J. Phys.: Condens. Matter 24, 155701 (2012).


New paper on H2S+H2 accepted in PRL

November 2, 2011

In a combined experimental / theoretical effort, our new new paper on H2S+H2 was just accepted for publication in Physical Review Letters. In this work we demonstrate the formation of a "guest-host" type molecular compound. Hydrogen bonding strength is tuned with pressure until the formation of new clathrate structure occurs.

Many of water’s familiar properties are due to hydrogen bonding: the attractive interactions that exist between molecules due to their polarity. For its molecular weight, water (H2O) shows extremely high melting and boiling points, as well as a wide temperature range separating the two properties.In the presence of small molecules, water crystalizes into compounds called clathrates. These compounds, similar to traditional ice, trap small “guest” molecules inside polyhedral hydrogen-bonded “host” water cages. Clathrate compounds trap small hydrocarbons naturally on the ocean floor, and can even be used to store and transport gases like hydrogen for future energy applications.

While chemically similar to water, hydrogen sulfide (H2S) shows little to no hydrogen bonding under normal conditions (as evidenced by its low melting and boiling temperatures); however, for the pure component, pressure can be used to tune intermolecular interactions and increase the structural significance of hydrogen bonding.

In a combined experimental and theoretical effort, researchers from the Geophysical Laboratory    (Timothy A. Strobel, Maddury Somayazulu and Russell J. Hemley) and Oak Ridge National Laboratory (Panchapakesan Ganesh and Paul R. C. Kent) have discovered a new compound formed from mixtures of H2S and H2. The constituents of the new compound, (H2S)2H2, interact through weak van der Waals forces in the low-pressure molecular alloy. With increasing pressure, hydrogen-bonding develops and strengthens between neighboring H2S molecules, and at 17 GPa, molecules order themselves into a hydrogen-bonded clathrate structure, similar to those observed for H2O.

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For the first time researchers have demonstrated how interactions in molecular compounds may be tuned from the weak hydrogen-bonding limit into the structurally ordered regime. This research may now open the door for an entirely new class of clathrate structures based on hydrogen-bonded H2S frameworks.

This study can be found at: T.A. Strobel et al., Phys. Rev. Lett. 107, 255503 (2011).


Oleksandr Kurakevych joins the lab

October 17, 2011

Oleksandr Kurakevych arrived at the Geophysical Laboratory on October 18, 2011 from the Institute for Mineralogy and Condensed Matter Physics, Université Pierre et Marie Curie (Paris 6) in France. He specializes in high-pressure chemistry and joins us as a postdoctoral associate. Oleksandr will be working on the synthesis and characterization of novel carbon-based compounds which are expected to exhibit remarkable materials properties.

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Strobel recieves 2011 Jamieson Award in Mumbai

September 26, 2011

Carnegie's Geophysical Laboratory's newest staff member, Timothy Strobel, will be given the prestigious Jamieson Award on September 26, 2011, from the International Association for the Advancement of High Pressure Science and Technology in Mumbai, India. The Jamieson Award is given to a scientist who has just completed outstanding PhD thesis research or to an exceptional postdoctoral researcher. Strobel's research focuses on developing new hydrogen-based materials to meet our country's energy challenges. "Tim began his work at Carnegie as fellow in 2008," remarked Geophysical Laboratory director Russel Hemley. "His novel approach to exploring and creating new materials exemplifies Andrew Carnegie's vision of supporting exceptional individuals. As a staff scientist, Tim will be expanding his research on materials in new directions." Strobel is being honored for his studies of how hydrogen can be stored in tiny molecular cages called clathrates and in related compounds. His work, which has been published in a number of prestigious journals, has already received over 300 citations. Strobel received his PhD in chemical engineering from the Colorado School of Mines in 2008. Last year he was honored with the 2010 Young Investigator Lecture at the Gordon Conference on High Pressure Research. He became a Carnegie staff scientist at the Geophysical Laboratory on September 1, 2011. "Carnegie is proud to support young investigators with a vision of the future," remarked Carnegie president Richard Meserve. "We look forward to Tim's future successes." The International Association for the Advancement of High Pressure Science and Technology is also known as Association Internationale pour L'Avancement de la Reserche et de la Technologie aux Hautes Pression (AIRAPT). The organization holds international conferences every two years to promote high-pressure science and technology. Strobel will be presented the Jamieson Award and deliver the award lecture at the September meeting in Mumbai.

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