The surface of our earth only exposes a fraction of matter the planet is made of, and most of it lies below the earth’s crust, where the pressure and temperatures differ significantly from ambient conditions. At the core of the earth, the pressures are as high as 360 GPa, more than three million times the atmospheric pressure. High-pressure experiments aim at creating such conditions in the laboratory and are essential to understand how materials behave under extreme pressures.
To achieve such high pressures, it is necessary to apply large forces on a small area. In a diamond anvil cell (DAC), two diamonds are placed opposite of each other with carefully polished tips (culets), forming two tiny surfaces opposing each other. Between these culets, a small sample of a material can be placed together with a pressure medium. By pressing the two diamonds against each other, a pressure of up to 400 GPa can be achieved. Using such a device it is possible to study matter at the conditions of the planet’s core. Since the diamonds are transparent to a wide range of radiation, it is possible to “look” trough them at the sample and observe how a material behaves at different pressure, allowing for example in-situ X-ray diffraction, Raman and Mössbauer measurements. Furthermore, the sample can be studied with respect to its electrical properties by attaching electrodes, for example to investigate the drop in resistivity in superconductors.
Recently, high pressure experiments have become increasingly popular as a method to synthesize new materials. Compounds that have been thought to be impossible to form based on chemical intuition can emerge once a sufficiently high pressure is applied. A special class of such materials are composed of elements that are immiscible at ambient conditions, i.e. of elements that do not form any chemical bonds at atmospheric pressures. However, at high pressures new compounds can be forged with exotic structural, chemical and physical properties. I have been involved in the prediction of such materials in ambient-immiscible systems, for example in Cu-Bi where we were able to predict and synthesize at least two new compounds at pressures above 3 GPa. Click here for the full story.