From the weather to air travel, pressure plays a dramatic role in our lives. It plays especially important roles during chemical reactions. By manipulating pressure, chemists can force chemical reactions to occur and the transitions between solids, liquids, and gases to accelerate. However, these pressure manipulations are on a small scale, not differing that much from the atmospheric pressure we experience everyday. Pressures elsewhere in the universe are billions of times greater and their influence on chemistry remains virtually unknown [1-5]. By researching how different compounds react at these pressures, scientists can discover and develop new materials that are more energy efficient and durable. This potential for discovery exists because chemicals can change dramatically at extremely high pressures. For instance, carbon is used for a variety of applications, including the graphite in pencil leads and as a building material, but under extreme pressures it forms diamond. Oxygen, normally a colorless gas, becomes a brilliant scarlet solid, and sulfur becomes an electrical conductor when exposed to high pressure [1]. Currently, scientists are attempting to study the influence of these pressures in order to develop novel materials and understand the chemistry occurring at the center of planets and stars.

What is high pressure chemistry and how is it conducted?

When studying the influence of high pressures on chemistry, scientists use two tools: Simulations and Diamond-Anvil Cells (DACs) [1-5]. They use computer simulations to try to predict the properties of many materials under high pressure. These studies provide insight into high pressure chemistry, but they rely on information obtained at normal atmospheric pressure. Consequently, some predictions obtained in these simulations are inaccurate when tested in the lab. To determine if the predictions are accurate, scientists constructed a machine known as a DAC (Figure 1). The DAC forces two diamonds together, crushing a material of interest and creating almost 10,000 times atmospheric pressure at room temperature [1-5]. Together, these tools have allowed scientists to determine the properties of many materials under extreme pressure.

Figure 1. A diamond-anvil cell. A material held in place by putty is put under pressure between two diamonds. Photo by Roy Kaltschmidt for Lawrence Berkeley National Laboratory (http://newscenter.lbl.gov/news-releases/2012/12/13/nanocrystals-not-small-enough-to-avoid-defects/).

One recent study using high pressure chemistry has revealed surprising information about table salt. Work from Zuzana Konopkova’s group at China Agricultural University has shown that Sodium Chloride (NaCl) forms solids containing unusual ratios of sodium and chlorine under high pressure conditions in a DAC [2]. Normally, sodium (Na) and chlorine (Cl) react to form NaCl in a 1 Na:1 Cl ratio across a wide range of conditions. However, Konopkova’s group has found that sodium chloride can exist in ratios of 2:1, 3:2, 1:7, and 1:3 [2]. These compounds with abnormal ratios appear to violate all known laws of chemical physics, and some of these ratios are stable even after high pressure has been removed [2]. Furthermore, unlike normal table salt, these compounds are metals and able to conduct electricity [2]. Although the potential utility of these compounds remains to be determined, chemists are hopeful that studying substances with DACs could lead to the discovery of materials useful as fuels and energy storage devices [2].

Applications of high pressure chemistry

High pressure chemistry is becoming increasingly important for the production of alternative fuel sources. Recently, French scientists discovered that mixing water, a mineral called olivine, and aluminum oxide in a DAC and subjecting the mixture to extreme pressure for 24 hours creates hydrogen gas [3,4]. This method has yet to be performed on a massive scale but could serve as a robust source of hydrogen for hydrogen fuel cells that could be used to power automobiles [3,4]. However, to power these fuel cells, hydrogen must be stored efficiently. Storing hydrogen as a liquid or gas actually consumes more energy than the hydrogen can produce. Using high pressure chemistry, it may be possible to create an energy efficient hydrogen storage material. Indeed, chemists in the United Kingdom have developed a method to synthesize Iridium hydride, a hydrogen storage material capable of producing more energy than was used in its creation [5]. High pressure chemistry can also be used to improve solar power. Recently, scientists at the Carnegie Institute have produced a mixture of silicon and sodium that, when combined under high pressure, becomes an excellent material for use in solar panels even after pressure is removed [1]. These applications are just a few small examples of what may be possible using high pressure chemistry.

Aside from technology development, high pressure chemistry can also be used to learn more about the composition of planets and stars. Most of our knowledge of the earth’s core and the core of stars is theoretical, however, using DACs, scientists can create the pressures seen in these stars. In fact, Russell J. Hemley’s group at the Carnegie Institute for Science has shown that when hydrogen gas is compressed it forms a material similar to that of graphene or pencil lead [1]. Since hydrogen is the most abundant element in the universe, this research could lead to a deeper understanding of the cores of stars and planets. NASA is already launching a mission to study Jupiter and this work may corroborate the data they obtain [1].

The study of high pressure chemistry has tremendous potential. However, mass-producing the types of materials created in DACs remains a challenge. Consequently, these technologies will not be part of our everyday lives for many years. Nevertheless, understanding this type of chemistry will allow the generation of novel materials that can serve as alternative energy sources and allow us to gain a deeper understanding of our universe.

Jacob Layer is a PhD candidate in the Biological and Biomedical Sciences program at Harvard University

References

[1] Chang K. (2013) The Big Squeeze, in the New York Times, New York. http://www.nytimes.com/2013/12/17/science/the-big-squeeze.html?pagewanted=all&_r=0
[2] (2013) ‘Impossible’ Sodium Chlorides Challenge Foundation of Chemistry http://www.sci-news.com/othersciences/chemistry/science-sodium-chlorides-foundation-chemistry-01633.html
[3] (2013) Deep Carbon Observatory scientists discover quick recipe for producing hydrogen http://www.spacedaily.com/reports/Deep_Carbon_Observatory_scientists_discover_quick_recipe_for_producing_hydrogen_999.html
[4] (2013) Hydrogen Fuel Becomes More a Reality http://www.earthtechling.com/2013/12/hydrogen-fuel-becomes-more-a-reality/
[5] Hadlington S. (2013) Pressure cooker produces new metal hydride http://www.rsc.org/chemistryworld/2013/11/pressure-cooker-iridium-hydride
[6] Timmer J. (2013) Strange, unpredictable chemistry at high pressure, Ars Technica. http://arstechnica.com/science/2013/09/strange-unpredictable-chemistry-at-high-pressure/