George Wolf

Most of our present knowledge of chemistry has been accrued over only a small window in the parameters which can influence chemical behavior. With recent advancements in experimental techniques it is now possible to study chemistry at conditions that are far removed from those of our normal environment. In our group we exploit pressure and temperature as a means to both statically and dynamically alter the nuclear separations and structural geometry in materials, and hence, can produce dramatic changes in a material’s chemical and physical properties. For example a solid that is an insulator at zero pressure can become a semiconductor, a metal, or even a superconductor at high pressures. Often a crystalline solid undergoes radical changes in its atomic structure at high pressure or may even convert to an amorphous state. It is sometimes possible to metastably trap these high-energy states upon decompression back to ambient conditions, a powerful means to synthesize novel new materials. Spectroscopic measurements under controlled pressure and temperature conditions also provide new insight into the dynamical processes of materials and the mechanisms of phase transitions or chemical reactions. Moreover, pressure is a powerful variable in exploring the interatomic force fields of materials. Measurements of the change in structure, dynamics and electronic properties up to extreme conditions of pressure and temperature provide critical tests for theoretical models of bonding.

Central to our experimental research is the high-pressure diamond anvil cell. The incredible advancements in high-pressure research over the last decade can be directly attributed to the development of the diamond anvil cell. With this device, materials can be exposed to near hydrostatic pressures in excess of a megabar (one million times the atmospheric pressure at the surface of the Earth). The powerful utility of this high-pressure cell arises from the unique properties of diamond. Not only is diamond the strongest known material, it is also transparent to a wide range of electromagnetic radiation. Infrared lasers are used to heat samples under pressure in the diamond anvil cell to temperatures in excess of 4000K. Optical lasers are used for Raman and Brillouin spectroscopy to study the vibrational and elastic properties of materials at high pressure. X-rays from synchrotron sources are used to study the high pressure and high temperature structural properties of materials. NMR is used to provide information on both the structural and dynamical properties of liquids over an extreme range in density. But, perhaps the most appealing advantage is that a sample at high pressures can be visually observed in the diamond cell under a microscope.