Materials Under High Pressures

Schematic diagram of a piston-cylinder diamond-anvil cell. Biphenyl in the twisted (top) and planar (bottom) conformations.

Diamond-anvil cell

Pressure is an experimental parameter that can be tuned over a wide range to probe the properties of condensed-matter systems. One tool for applying large, static pressures is a diamond anvil cell. In our laboratory, we have constructed a piston-cylinder diamond-anvil cell for infrared (IR) spectroscopy. Pressure is applied by tightening six Allen screws. The diamonds have flat, 700-micron diameter culets, that press against a stainless-steel gasket. A small hole in the gasket contains the sample, along with pressure-transmitting fluid such as liquid nitrogen. Pressures up to 100,000 atmospheres are routinely produced in our lab.

A parabloic mirror focuses the IR beam through the first diamond and onto the sample. The beam then travels through the second diamond and is collected by a very sensitive detector. The Ge:Cu photoconductor detector was provided by Jeff Beeman and Eugene Haller at Lawrence Berkeley National Laboratory. The entire assembly is placed in a liquid-helium cryostat. Our measurements are performed at very low temperatures (4-10 K).

Molecular solids under pressure

We performed a systematic study of organic conjugated molecules (biphenyl, terphenyl, and quaterphenyl) under pressure. A schematic diagram of biphenyl is shown in the figure above, where the hexagons are phenyl rings. At low pressures (and low temperatures), the molecule is twisted. At high pressures, the molecule flattens into a planar conformation. IR spectroscopy is an ideal method for studying the flattening of organic molecules. We have discovered that when the molecule flattens, certain IR peaks abruptly disappear. Current research is focused on pressure-induced phase transitions in pentacene, an important material for organic thin-film devices.

Confocal microscopy

We are currently using confocal microscopy to obtain high-resolution, 3D image reconstructions of solids being squeezed to high pressures. These pictures will enable us to obtain accurate pressure-volume relations for a range of materials. The figure below shows the pressure-induced freezing of water at room temperature (~1 GPa). The arrow indicates an ice crystal near the top of the gasket hole.

Semiconductors under pressure

The electronic, structural, and vibrational properties of semiconductors under pressure are areas of great interest. Pressure can be used to simulate alloying, as in the case of AlGaAs and AlGaN. We are interested in the effect of pressure on the local vibrational modes of impurities in semiconductors. By varying the pressure, we can obtain insight into impurity-host interactions. In silicon, for example, interstitial oxygen buckles under pressure, transforming from a harmonic oscillator to a rotor.  By using pressure as an experimental probe, we have tuned the oxygen local mode into resonance with an extended mode.  Such an interaction is important in the decay of local vibrational modes into lattice phonons.

Work supported by NSF and ACS-PRF.

Relevant publications