Diamond anvil cell
A diamond anvil cell (DAC) is a device used in scientific experiments. It allows compressing a small (sub-millimeter sized) piece of material to extreme pressures, which can exceed 600 gigapascals (6,000,000 bars / 6 million atmospheres)
The device has been used to recreate the pressure existing deep inside planets, creating materials and phases not observed under normal conditions. Notable examples include the non-molecular ice X, polymeric nitrogen and metallic xenon (an inert gas at lower pressures).
A DAC consists of two opposing diamonds with a sample compressed between the culets (tips). Pressure may be monitored using a reference material whose behavior under pressure is known. Common pressure standards include ruby fluorescence, and various structurally simple metals, such as copper or platinum. The uniaxial pressure supplied by the DAC may be transformed into uniform hydrostatic pressure using a pressure transmitting medium, such as argon, xenon, hydrogen, helium, paraffin oil or a mixture of methanol and ethanol. The pressure-transmitting medium is enclosed by a gasket and the two diamond anvils. The sample can be viewed through the diamonds and illuminated by X-rays and visible light. In this way, X-ray diffraction and fluorescence; optical absorption and photoluminescence; Mössbauer, Raman and Brillouin scattering; positron annihilation and other signals can be measured from materials under high pressure. Magnetic and microwave fields can be applied externally to the cell allowing nuclear magnetic resonance, electron paramagnetic resonance and other magnetic measurements. Attaching electrodes to the sample allows electrical and magnetoelectrical measurements as well as heating up the sample to a few thousand degrees. Much higher temperatures (up to 7000 K) can be achieved with laser-induced heating, and cooling down to millikelvins has been demonstrated.
The operation of the diamond anvil cell relies on a simple principle:
where p is the pressure, F the applied force, and A the area.
Therefore high pressure can be achieved by applying a moderate force on a sample with a small area, rather than applying a large force on a large area. In order to minimize deformation and failure of the anvils that apply the force, they must be made from a very hard and virtually incompressible material, such as diamond.
The study of materials at extreme conditions, high pressure and high temperature uses a wide array of techniques to achieve these conditions and probe the behavior of material while in the extreme environment. Percy Williams Bridgman, the great pioneer of high-pressure research during the first half of the 20th century, revolutionized the field of high pressures with his development of an opposed anvil device with small flat areas that were pressed one against the other with a lever-arm. The anvils were made of tungsten carbide (WC). This device could achieve pressure of a few gigapascals, and was used in electrical resistance and compressibility measurements. The principles of the DAC are similar to the Bridgman anvils but in order to achieve the highest possible pressures without breaking the anvils, they were made of the hardest known material: a single crystal diamond. The first prototypes were limited in their pressure range and there was not a reliable way to calibrate the pressure.
Following the Bridgman anvil, the diamond anvil cell became the most versatile pressure generating device that has a single characteristic that to this day sets apart from the other pressure devices. This provided the early high pressure pioneers with the capability to directly observe the properties of a material while under pressure. With just the use of a microscope, phase boundaries, color changes and recrystallization could be seen immediately without the collect of x-ray diffraction or spectroscopic measurements and their subsequent analysis. The potential for the diamond anvil cell was realized by Alvin Van Valkenburg while he was preparing a sample for IR spectroscopy and was checking the alignment of the diamond faces.
The diamond cell was created at the National Bureau of Standards (NBS) by Charles E. Weir, Ellis R. Lippincott, and Elmer N. Bunting. Within the group each memeber focused on different applications of the diamond cell. Van focused on making visual observations, Charles on XRD, Ellis on IR Spectroscopy. The group was well established in each of their techniques before outside collaboration kicked off with university researchers like William A. Bassett and Taro Takahashi at the University of Rochester.
During the first experiments using diamond anvils, the sample was placed on the flat tip of the diamond, the cutlet, and pressed between the diamond faces. As the diamond faces were pushed closer together, the sample would be pressed and extrude out from the center. Using a microscope to view the sample, it could be seen that a smooth pressure gradient existed across the sample with the outer most portions of the sample acting as a kind of gasket. The sample was not evenly distributed across the diamond cutlet but localized in the center due to the "cupping" of the diamond at higher pressures. This cupping phenomenon is the elastic stretching of the edges of the diamond cutlet, commonly referred to as the "shoulder height". Many diamonds were broken during the first stages of producing a new cell or any time an experiment is pushed to higher pressure. The NBS group was in a unique position where almost endless supplies of diamonds were available to them. Custom officials occasionally confiscated diamonds from people attempting to smuggle them into the country. Disposing of such valuable confiscated materials could be problematic given rules and regulations. A solution was simply to make such materials available to people at other government agencies if they could make a convincing case for their use. This became an unrivaled resource as other teams at the University of Chicago, Harvard University and General Electric entered the high pressure field.
During the following decades DACs have been successively refined, the most important innovations being the use of gaskets and the ruby pressure calibration. The DAC evolved to be the most powerful lab device for generating static high pressure. The range of static pressure attainable today extends to the estimated pressures at the Earth's center (~360 GPa).
There are many different DAC designs but all have four main components:
Relies on the operation of either a lever arm, tightening screws, or pneumatic or hydraulic pressure applied to a membrane. In all cases the force is uniaxial and is applied to the tables (bases) of the two anvils
Made of high gem quality, flawless diamonds, usually with 16 facets. They typically weigh 1/8 to 1/3 carat (25 to 70 mg). The culet (tip) is ground and polished to a hexadecagonal surface parallel to the table. The culets of the two diamonds face one another, and must be perfectly parallel in order to produce uniform pressure and to prevent dangerous strains. Specially selected anvils are required for specific measurements—for example, low diamond absorption and luminescence is required in corresponding experiments.
A gasket used in a diamond anvil cell experiment is a thin metal foil, typically 0.3 mm in thickness, which is placed in between the diamonds. Desirable materials for gaskets are strong, stiff metals such as rhenium or tungsten. Steel is frequently used as a cheaper alternative for experiments not going to extreme pressures. The above mentioned materials cannot be used in radial geometries where the x-ray beam must pass through the gasket. They are not transparent to X-rays, and thus if X-ray illumination through the gasket is required then lighter materials, such as beryllium, boron nitride, boron or diamond are used as a gasket.
Gaskets are preindented using the diamonds and a hole is drilled in the center of the indentation. By creating this confined space the sample can be immersed in the fluid while under pressure. The sample chamber created by the gasket also allows for liquids and gasses to be studied under pressure.
The pressure transmitting medium is the compressible fluid that fills the sample chamber and transmitting the applied force to the sample. Hydrostatical pressure is preferred for high pressure experiments because variation in strain throughout the sample can lead to distorted observations of different behaviors. In some experiments stress and strain relationships are investigated and the effects of non-hydrostatic forces are desired. A good pressure medium will remain a soft, compressible fluid to high pressure.
- Gasses: He, Ne, Ar,
- Liquids: 4:1 Methanol/Ethanol, Silicone Oil, Fluorinert, Daphne 7474 Cyclohexane
- Solids: NaCl
The full range of techniques that are available has been summarized in a tree diagram by William Bassett. The ability to utilize any and all of these techniques hinges on being able to look through the diamonds which was first demonstrated by visual observations.
The two main pressure scales used in static high pressure experiments are X-ray diffraction of a material with a known equation of state and measuring the shift in ruby fluorescence lines. The first began with NaCl, for which the compressibility has been determined by first principles in 1968. The major pitfall of this method of measuring pressure is that you need X-rays. Many experiments do not require X-rays and this presents a major inconvenience to conduct both the intended experiment and a diffraction experiment. In 1971, the NBS high pressure group was set in pursuit of a spectroscopic method for determining pressure. It was found that the wavelength of ruby fluorescence emissions change with pressure, this was easily calibrated against the NaCl scale.
Once pressure could be generated and measured it quickly became a competition for which cells can go the highest. The need for a reliable pressure scale became more important during this race. Shock-wave data for the compressibility's of Cu, Mo, Pd and Ag were available at this time and could be used to define equations of states up to Mbar pressure. Using these scales these pressures were reported: 1.2 Mbar in 1976, 1.5 Mbar in 1979, 2.5 Mbar in 1985, and 5.5 Mbar in 1987.
Both methods are continually refined and in use today. However, the ruby method is less reliable high temperature. Well defined equations of state are needed when adjusting temperature and pressure, two parameters that affect the lattice parameters of materials.
Prior to the invention of the diamond anvil cell, static high-pressure apparatus required large hydraulic presses which weighed several tons and required large specialized laboratories. The simplicity and compactness of the DAC meant that it could be accommodated in a wide variety of experiments. Some contemporary DACs can easily fit into a cryostat for low-temperature measurements, and for use with a superconducting electromagnet. In addition to being hard, diamonds have the advantage of being transparent to a wide range of the electromagnetic spectrum from infrared to gamma rays, with the exception of the far ultraviolet and soft X-rays. This makes the DAC a perfect device for spectroscopic experiments and for crystallographic studies using hard X-rays.
A variant of the diamond anvil, the hydrothermal diamond anvil cell (HDAC) is used in experimental petrology/geochemistry for the study of aqueous fluids, silicate melts, immiscible liquids, mineral solubility and aqueous fluid speciation at geologic pressures and temperatures. The HDAC is sometimes used to examine aqueous complexes in solution using the synchrotron light source techniques XANES and EXAFS. The design of HDAC is very similar to that of DAC, but it is optimized for studying liquids.
An innovative use of the diamond anvil cell is testing the sustainability and durability of life under high pressures. This innovative use can be used in the search for life on extrasolar planets. One reason the DAC is applicable for testing life on extrasolar planets is panspermia, a form of interstellar travel. When panspermia occurs, there is high pressure upon impact and the DAC can replicate this pressure. Another reason the DAC is applicable for testing life on extrasolar planets is that planetary bodies that hold the potential for life may have incredibly high pressure on their surface.
Anurag Sharma, a geochemist, James Scott, a microbiologist, and others at the Carnegie Institution of Washington performed an experiment with the DAC using this new innovative application. Their goal was to test microbes and discover under what level of pressure they can carry out life processes. The experiment was performed under 1.6 GPa of pressure, which is more than 16,000 times Earth’s surface pressure (Earth’s surface pressure is 985 hPa). The experiment began by placing a solution of bacteria, specifically Escherichia coli and Shewanella oneidensis, in a film and placing it in the DAC. The pressure was then raised to 1.6 GPa. When raised to this pressure and kept there for 30 hours, only about 1% of the bacteria survived. The experimenters then added a dye to the solution. If the cells survived the squeezing and were capable of carrying out life processes, specifically breaking down formate, the dye would turn clear. 1.6 GPa is such great pressure that during the experiment the DAC turned the solution into ice-IV, a room-temperature ice. When the bacteria broke down the formate in the ice, liquid pockets would form because of the chemical reaction. The bacteria were also able to cling to the surface of the DAC with their tails.
However, there is some skepticism with this experiment. People debate whether carrying out the simple process of breaking down formate is enough to consider the bacteria living. Art Yayanos, an oceanographer at the Scripps Institute of Oceanography in La Jolla, California, believes an organism should only be considered living if it can reproduce. Another issue with the DAC experiment is that when high pressures occur, there are usually high temperatures present as well, but in this experiment there were not. This experiment was performed at room-temperature, which causes some skepticism of the results.
Moving past the 10 years of skepticism, new results from independent research groups  have shown the validity of Sharma et al. (2002)  work. This is a significant step that reiterates the need for a new approach to the old problem of studying environmental extremes through experiments. There is practically no debate whether microbial life can survive pressures up to 600 MPa, which has been shown over the last decade or so to be valid through a number of scattered publications. What is significant in this approach of Sharma et al. 2002 work is the elegantly straightforward ability to monitor systems at extreme conditions that have since remained technically inaccessible. While the simplicity and the elegance of this experimental approach is mind-boggling; the results are rather expected and consistent with most biophysical models. This novel approach lays a foundation for future work on microbiology at non-ambient conditions by not only providing a scientific premise, but also laying the technical feasibility for future work on non-ambient biology and organic systems.
There is another group of scientists performing similar tests with a low-pressure diamond anvil cell. This low-pressure DAC has better imaging quality and signal collection. It is designed to sense pressures in the 0.1–600 MPa range, much lower than the high pressure DAC. The new low-pressure DAC also has a new asymmetric design, as opposed to a symmetric design the old, high pressure DAC used. In this experiment Saccharomyces cerevisiae is the microbe being observed. Saccharomyces cerevisiae is more commonly known as baker’s yeast. These microbes can only grow in pressures ranging from 15–50 MPa, while pressures over 200 MPa are likely to kill the cells. The microbes were also incubated at 30 °C. Their tests showed that the yeast completed its cell cycle in 97±5 minutes.
Single Crystal X-ray Diffraction
Good single crystal diffraction experiments in diamond anvil cells require sample stage to rotate on the vertical axis, omega. Most diamond anvil cells do not feature a large opening that would allow the cell to be rotated to high angles, a 60 degrees opening is considered sufficient for most crystals but larger angles are possible. The first cell to be used for single crystal experiments was designed by a graduate student at the University of Rochester, Leo Merrill. The cell was triangular with beryllium seats that the diamonds were mounted on; the cell was pressurized with screws and guide pins holding everything in place.
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