What the Heck is a Diamond Anvil Cell?

By Daniel Hummer, DCO Extreme Physics and Chemistry, Carnegie Institution of Washington, USA

In the Extreme Physics and Chemistry (EPC) community, our main goal is to learn about the structure, properties, and behavior of carbon-bearing materials at the extreme temperature and pressure conditions in the interior of Earth and other planetary bodies. Scientists have performed experiments at elevated temperatures for centuries, as there are many methods available for achieving high temperature. Attaining high pressure conditions in the laboratory, however, has historically proved much more problematic. High pressure experiments must take place in an enclosed space that is difficult to directly observe, and there are few techniques for producing the pressures found beneath Earth’s crust. The most widely used device for achieving very high pressures in a controlled laboratory setting is the diamond anvil cell (DAC).

Figure 1: Photo of a diamond anvil cell (DAC), with a blown up cross-section of the region where the experiment takes place. Depending on the DAC design, the diameter of the sample chamber is typically between 100 and 300 μm. Figure from Yang and Zhaohui [1].
Figure 1: Photo of a diamond anvil cell (DAC), with a blown up cross-section of the region where the experiment takes place. Depending on the DAC design, the diameter of the sample chamber is typically between 100 and 300 μm. Figure from Yang and Zhaohui [1].
A DAC consists of two round, metal plates on which two cut and polished diamonds are mounted (Fig. 1). The sample, as well as a standard reference material (see next section) is placed in a chamber with the diamonds on the top and bottom, and a gasket around the side. The diamonds act as anvils, pressing inward on the material between them. The gasket is a thin strip of metal, typically rhenium, with a hole drilled in the center. Note that the seats on which the diamonds sit (and the entire DAC assembly) have holes in the middle so that various beams of radiation, including X-ray diffraction and many types of spectroscopy, can be sent through the diamonds and into the sample chamber to probe the properties of whatever sample material is being studied. Several characteristics make diamond the ideal anvil material for in situ high-pressure experiments, including transparency across a broad range of the electromagnetic spectrum, chemical inactivity, stability at very high pressures, and extreme hardness.

The first researcher to implement the anvil design that modern DACs are based on was Percy Bridgman, a high pressure physicist at Harvard University [2]. His high pressure device, and the many discoveries it enabled, earned him the 1946 Nobel Prize in physics. The mineral that dominates Earth’s mantle, whose discovery would not have been possible without Bridgman’s work, was recently and very fittingly named bridgmanite in his honor [3], in place of its previous designation of “perovskite”. Nowadays, multiple DAC designs are available depending on the desired pressure range and analytical technique.

Before starting an experiment, we must load a “pressure medium” into the chamber to shield the sample from contact with the diamonds and gasket, and distribute the pressure evenly throughout the chamber. Because they are easily compressible and chemically inert, noble gases such as He, Ne, and Ar are often employed for this purpose, although other gases or even liquid water are occasionally used [4]. In order to enclose the samples and pressure medium within the sample chamber in an airtight fashion, we carefully put the samples, gasket, and diamonds in place, but we leave one diamond at a slight distance away from the gasket. Then, in a process called “gas loading,” we place the entire DAC in a gas-filled box so that the desired pressure medium can infiltrate the sample chamber. We then tighten the screws to close the gap between the diamonds and gasket, sealing the experimental chamber from the surrounding environment.

We can apply more pressure by turning the screws at the edge of the DAC, which forces the diamonds closer together. The secret of generating such high pressures is to take advantage of the fact that pressure is force per unit area: P = F/A. Although the screws apply only a moderate force, the flat surfaces of the diamond which face the sample chamber, called “culets”, have such a tiny area that very large pressures can be generated. The exact same principle makes ice-skating possible – a person transmits the entire weight of their body to the very small surface area of the blades, generating enough pressure to melt the ice under the blade and enable lubricated motion across the surface. A well-constructed and aligned DAC can produce pressures in excess of 300 GPa, close to the pressure of Earth’s inner core (Fig. 2) [5,6]. With special setups such as a double-stage, even higher pressures can be achieved – the highest recorded so far is an astounding 770 GPa, twice the pressure at the center of Earth [7]! However, in routine practice, increasing the pressure becomes rapidly more difficult once you surpass ~100 GPa.

Figure 2: A cross section of the Earth showing the major layers, and the depths and pressures corresponding to their boundaries. The center of the Earth is thought to be ~ 364 GPa. Figure from [6].
Figure 2: A cross section of the Earth showing the major layers, and the depths and pressures corresponding to their boundaries. The center of the Earth is thought to be ~ 364 GPa. Figure from [6].
How the heck do you know the pressure?

To perform a useful experiment, we need to independently measure the pressure the sample is experiencing. To do this, we must place in the sample chamber a reference material with a measurable property that responds to pressure in a well-known fashion. High-pressure researchers refer to this as a “pressure calibration” or “pressure scale”. There are two widely used methods for measuring pressure: ruby fluorescence, and the equation of state of a reference material.

Since rubies contain Cr3+ ions surrounded by oxide ions, electronic transitions among the Cr3+ valence electrons produce a fluorescent emission at about 694 nm at room pressure [8]. When the ruby is placed under pressure, the oxide ions move closer to the Cr3+ ions, systematically changing the electron energy levels and therefore shifting the wavelength of the fluorescence line. Thus, by shining a laser through a DAC onto a ruby crystal and measuring the wavelength of fluorescence, we can calculate the pressure inside our sample chamber. Mao et al. [8,9] were the first to calibrate how ruby fluorescence changes with pressure in 1978, and this calibration is still routinely used for pressures below ~60 GPa.

Figure 3: The intensity of ruby fluorescence vs. wavelength at two difference pressures within a DAC. Since the position of the peak maximum is very sensitive to pressure, the wavelength of the fluorescence line is useful as a pressure scale. Figure from Mao et al. [9].
Figure 3: The intensity of ruby fluorescence vs. wavelength at two difference pressures within a DAC. Since the position of the peak maximum is very sensitive to pressure, the wavelength of the fluorescence line is useful as a pressure scale. Figure from Mao et al. [9].
The other method for measuring pressure within a DAC uses the equation of state (EOS) of a reference material. The EOS of a material describes the relationship between its state variables: pressure, volume, and temperature (or in the case of an experiment at room temperature, just pressure and volume). If the volume decrease of a reference material is already well known as a function of pressure, then measuring the volume of that material within the DAC provides a measurement of pressure. In practice, volume is usually measured via the positions of X-ray diffraction peaks of a crystalline reference material. However, choosing a reference material requires care: it must not react with the sample or pressure medium at the experimental conditions, and the pressure-volume relationship must be well calibrated across the entire pressure range of the experiment.

The EOS of a variety of materials have been calibrated, but the most commonly used in DAC experiments are gold [10,11], platinum [12,13], magnesium oxide [14,15], sodium chloride [16,17], and neon [18]. Unfortunately, these pressure scales are not fully consistent with each other, making detailed comparison of experiments using different pressure scales cumbersome. Recently, a number of researchers have attempted to cross-calibrate and reconcile these various pressures scales [19-21].

Figure 4: The equation of state of MgO is plotted in volume-pressure space, with different curves representing different temperatures. Thus, a volume measurement of MgO within a DAC can be converted to pressure. Figure from Tange et al. [15]
Figure 4: The equation of state of MgO is plotted in volume-pressure space, with different curves representing different temperatures. Thus, a volume measurement of MgO within a DAC can be converted to pressure. Figure from Tange et al. [15]
How the heck do you heat stuff?

I have focused so far on generating and measuring high pressures, but since Earth scientists often want to measure sample properties that are a function of both pressure and temperature, it is crucial to be able to heat samples to a known temperature while under pressure within a DAC. There are two methods for doing this – a heating jacket and laser heating.

A heating jacket is a metal jacket placed around the circumference of a DAC, through which super-heated water is pumped, heating the entire DAC assembly. Because we can only heat water to approximately 200 oC in an enclosed environment, this heating method is somewhat limited [22]. However, the temperature stability and precision is better than with laser heating.

In order to heat a sample to thousands of degrees, we have to use laser heating. By aiming a high intensity laser beam through the diamond windows at the sample, the sample can heat to temperatures well above 3000 K through absorption of the radiation [23]. The intensity of the laser controls sample heating, and by collecting the blackbody spectrum and fitting it to Plank’s law, we know the sample temperature. However, strong thermal gradients within the sample chamber and uncertainties in fitting the blackbody spectrum often produce large error bars for temperature measurements made with this method. Techniques for improving thermal stability, minimizing thermal gradients, and maximizing accessible temperatures in laser heating are still active areas of research [24,25].

What the heck can you measure in a diamond anvil cell?

A wide variety of Earth materials can be placed under high pressure in a DAC, and many discoveries about Earth’s interior (and the interiors of other planetary bodies) have been made through DAC experiments. While a full accounting of techniques and results from DAC experiments would take volumes, we can briefly highlight here the types of measurements that are often made.

A common type of experiment is to place a mineral or other composition that we know is abundant in Earth under pressure, and look for phase transitions (i.e., rearrangements of the atomic structure that would produce a new mineral). This is typically done by shining monochromatic X-rays onto the sample, and looking for sudden changes in the X-ray diffraction pattern as pressure is increased. For example, by placing the aforementioned mantle mineral “perovskite” (now called bridgmanite [3]) under conditions of ~125 GPa and ~2500 K, two independent teams found a change in crystal structure producing a new material called “post-perovskite”. [26,27] This discovery helped explain unusual seismic observations in the lowermost part of the mantle.

 Figure 5: Illustration of the phase transition in MgSiO3 from the perovskite structure (a) to the post-perovskite structure (b) when pressurized above 125 GPa. Gray octahedra represent SiO6 groups, and red spheres represent Mg. Figure from [28].
Figure 5: Illustration of the phase transition in MgSiO3 from the perovskite structure (a) to the post-perovskite structure (b) when pressurized above 125 GPa. Gray octahedra represent SiO6 groups, and red spheres represent Mg. Figure from [28].
It is also routine to measure the EOS of newly discovered Earth materials, which simply involves measuring the material’s volume as a function of pressure. Such experiments tell us about the density of different layers in the Earth. EOS measurements are typically made via X-ray diffraction, which can be used to track the decrease in interatomic distances in crystals [10-21], but volume measurements of the entire sample can now also be made directly using X-ray tomography [29]. Sometimes, abrupt changes in volume can be seen even without a change in atomic structure. These are often the result of changes in the electronic configuration of the material, which are referred to as “spin transitions”. These can be examined more precisely by irradiating the sample with high energy X-rays, and measuring the wavelengths of emitted X-rays (i.e., X-ray emission spectroscopy) [30,31].

With special setups, other material properties can be measured, such as thermal conductivity [25], electrical conductivity [32], and magnetization [33]. Such experiments inform us about how heat flows from one part of the Earth to another, and how the Earth’s magnetic field is generated. When fluids such as water are placed in a DAC, various spectroscopic techniques can be used to measure the solubility of minerals in the fluid as a function of pressure [34], and learn about the arrangement of atoms in the fluid [35].

By using a variety of combinations of DAC models, pressure scales, analytical techniques, and heating methods, members of the EPC community are testing the properties of numerous carbon-bearing minerals and fluids at a wide array of deep Earth conditions. With new techniques continuously under development, be on the lookout for studies that push experimental boundaries and uncover processes occurring thousands of kilometers beneath our feet, because it’s wicked awesome stuff.

Who the heck would I like to acknowledge?

The author is very grateful for helpful comments from Sami Mikhail (University of St. Andrews, UK), Dana Thomas (Stanford University, USA), Katie Pratt (University of Rhode Island, USA), Peter Barry (University of Oxford, UK), and Karen Lloyd (University of Tennessee, USA) which greatly improved the content and presentation of this article.

Where the heck are those references?

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