diamond anvil cell

diamond anvil cell. A diamond anvil cell is a premier high-pressure apparatus used in physics, chemistry, and geoscience to subject tiny samples to pressures exceeding those at the center of the Earth. It functions by compressing a sample between the flattened tips, or culets, of two opposing gem-quality diamonds. This compact device enables in-situ study of material properties under extreme conditions using techniques like X-ray diffraction and Raman spectroscopy.

Design and components

The core of the device consists of two brilliant-cut diamond anvils, typically with culets ranging from 10 to 500 micrometers in diameter. These anvils are mounted in opposing pistons made of robust materials like tungsten carbide or beryllium copper. A pre-indented metal gasket, often fabricated from rhenium or stainless steel, is placed between the culets to contain the sample and a pressure-transmitting medium such as argon, helium, or sodium chloride. The entire assembly is held within a sturdy mechanical frame that allows for precise alignment and compression, with designs pioneered at institutions like the Carnegie Institution for Science.

Operating principles

Pressure is generated by mechanically forcing the two diamond anvils together, which focuses an immense force onto the minuscule sample area confined within the gasket hole. According to the principles defined by Blaise Pascal, this force per unit area translates to extraordinarily high pressures. The exceptional hardness and strength of diamond, coupled with its transparency across a broad spectrum from infrared to X-ray regions, are fundamental to the cell's operation. This transparency permits a wide array of spectroscopic and diffraction probes to analyze the sample without significant interference.

Pressure generation and measurement

The device can routinely achieve static pressures exceeding 300 gigapascals, with record experiments surpassing 700 GPa. Pressure is most commonly calibrated using the fluorescence shift of a tiny chip of ruby placed within the sample chamber, a technique developed by researchers at the National Institute of Standards and Technology. Alternative calibrants include the fluorescence of yttrium aluminum garnet doped with samarium or the known equations of state of standards like gold or platinum. The precise force applied is measured via a load cell or calibrated screws, while the pressure distribution is monitored using Raman spectroscopy of the diamond stress itself.

Scientific applications

The diamond anvil cell has been instrumental in numerous groundbreaking discoveries. In geophysics, it has been used to recreate and study the conditions within Earth's core, leading to insights into the composition of the inner core and the properties of silicate minerals in the lower mantle. It enabled the synthesis of metallic hydrogen, a long-predicted state of matter, and the discovery of high-temperature superconductivity in hydrogen sulfide and lanthanum hydride under pressure. Furthermore, it is a critical tool in materials science for creating novel phases, such as super-hard forms of nitrogen or carbon dioxide, and in chemistry for probing reaction pathways under extreme conditions.

Limitations and challenges

Primary challenges include the small sample size, often picoliters in volume, which complicates certain analyses and can lead to large pressure gradients. Achieving truly hydrostatic conditions becomes difficult above ~50 GPa as most media solidify, inducing shear stresses. The diamonds themselves have limits; they can fracture under extreme shear or undergo a phase transition to a graphite-like form under non-hydrostatic stress at the culet edges. Furthermore, heating samples to high temperatures, often using laser heating systems like those at the Advanced Photon Source, introduces complexities in temperature measurement and control. Despite these hurdles, ongoing advancements in gasket design, synchrotron techniques, and dynamic compression methods continue to expand the frontiers of high-pressure research.

Category:Scientific equipment Category:High-pressure physics Category:Laboratory techniques