Diamond anvil cell

A tool for high pressure and high temperature

The Diamond anvil cells (DAC) are devices that allow reaching extreme pressures. It is relatively easy to reach pressure in the order of the megabar using a DAC. The Diamond is the hardest material however, it is also very brittle. Therefore, it is necessary to handle them with great care in order to prevent breaking them.

There is a large variety of DAC designs. They are either purchase through companies or custom made in the university machine shop. The shape and design of the cell depends upon the choice of analytical method. There are several types of diamonds. In particular, the fluorescence and color of the diamond differs a lot from one stone to the next. It is necessary to select a particular diamond for a specific experiment in order to optimize the result.

Symmetrical Mao Type Diamond anvil cell designed for Laser heating at extreme pressure. Such cell can easily access 100 GPa.

During a diamond anvil cell experiment, the sample is placed in a hole in a metal gasket. The choice of the metal depends upon the experiment. The most common gasket materials are stainless steel, rhenium and tungsten. Other materials such as boron epoxy mixture and beryllium are also employed for some very specific types of experiments where a transparent gasket is necessary.

The gasket is first pre-indented by pressing it between the diamond anvils. This process works the gasket material and reduces the chance or its failure during compression.

Animation showing the indentation and drilling of the gasket followed by the loading compression and probing of the sample.

Pressure determination
The pressure determination in the diamond anvil cell can be performed in various way. Either using a "primary" scale or a derived pressure scale.

In order to use a primary scale, i.e. the equation of state of a material, it is necessary to have access to an intense X-ray source. In most cases, the primary pressure scale is only used when performing an experiment at a synchrotron. In that case, the X-ray diffraction pattern of the pressure calibrant is recorded at each pressure point. The main candidates for a primary pressure scale are Pt, Au, MgO, NaCl. However, in principal, one could use any material with a known equation of state (tool link).

The use of a secondary pressure scale is much more commun. The most spread method is using the pressure shift of the fluorescence lines from ruby
(tool link). The shift as a function of pressure has be calibrated well above 1 MBar (100 GPa). This method do not require the use of a synchrotron. The ruby crystal placed in the sample chamber is excited with a laser and its fluorescence spectrum is recorded with a spectrometer. This method is extremely efficent and simple.

Heating in the DAC
There are several way to heat a sample placed in a diamond anvil cell.

  • resistive heating: a small heater is placed around the gasket
  • Laser heating: a laser is focussed on the sample through the diamonds

  • References For more general details on the principal and how to begin to use a diamond anvil cell one care refer to the following reference.

    An introduction to diamond anvil cells and loading techniques
    Emmanuel Soignard, Paul F. McMillan
    NATO-ASI " Crystallography at High Pressure", Erice Crystallography Course/EuroSummerSchool, eds., A. Katrusiak, PF McMillan. Erice, Sicily, June 2003