Carnegie’s Innovations in High-Pressure Science

The new HPCAT sector will employ some of the “old” workhorses of high-pressure physics, such as diamond-anvil cells, in addition to the latest generation of advanced instrumentation, much of which has been developed by members of Carnegie’s high-pressure team. Beginning in the 1970s,the Geophysical Lab’s Ho-kwang Mao (now HPCAT team director) and Peter Bell used the diamond-anvil cell to break the 1-million-atmosphere mark—a milestone in high-pressure physics. Currently Russell Hemley, Mao, and colleagues are recognized as world leaders in the field, and new research using the HPCAT facility will propel the team even further.

Some of the more recent advances that have come from Carnegie scientists include a technique developed at APS sector 13, where questions in Earth science are the primary focus. This technique improves the way samples are subjected to extreme heat for diffraction studies. The standard method for extreme heating had been limited in that it could heat a sample from only one side using an infrared laser beam, which was cumbersome and prone to error. The Carnegie team invented a double-sided laser heating system, which includes improved temperature measurements. It has allowed researchers to obtain accurate temperatures to above 1000 K at pressures of millions of atmospheres. Working at Brookhaven National Laboratory, Carnegie scientists also pioneered the use of infrared microspectroscopy by taking advantage of the flux available from lower-energy synchrotron radiation sources so that new types of experiments can be conducted on extremely small samples at millions of atmospheres.

There are several unique features that have been developed specifically for the HPCAT facility, and with the sector nearing completion, these advances will make the facility the best and most flexible in the world for high-pressure/temperature experimentation. Four out of five experiment enclosures will be able to operate simultaneously. To accomplish this increase in productivity, the researchers devised a way to effectively double the X-ray beams, thus doubling the experiment capacity. They designed a system in which one wavelength of the X-ray is selected and channeled to one experiment enclosure for high-pressure spectroscopy measurements, while at the same time another wavelength is selected from the remainder of the radiation spectrum for high-pressure diffraction studies in another experiment enclosure. The instruments, called the Double Crystal Monochromator and the Branching Double Crystal Monochromator, exploit both the X-ray transparent and crystal optics properties of near-perfect diamonds to selectively divert the different wavelengths required for experiments. This doubling of the experimental capacity has been accomplished at minimal extra cost.

Even at a laserlike size, the beam is far too big for some of the high-pressure samples that scientists want to study. To study minute samples, the collaborators came up with novel ways of “demagnifying” the beam to produce X-ray beam sizes where the sample is positioned 10 to 40 times smaller than the size of the X-ray-emitting positron beam, which is the width of a human hair. They fashioned small mirrors into adjustable parabolic shapes that focus the X-rays to micron, and even submicron, sizes, and coupled this with ultra-accurate positioning systems for the optics and high-pressure equipment. The experiment and mirrors are located on separate high-stability tables that move on tracks. The track layout allows the sample in the pressure cell to be moved farther from or closer to the mirrors, thus changing the focal spot size to that needed for a given experiment. The team has also constructed a new system where two detectors can be used in the same experiment. In a parallel development in their lab in Washington, Hemley and Mao are currently developing ways to grow large diamonds (up to 25 carats as opposed to the 0.25 carat typically used today), which will be used in a new generation of anvil cells that will be able to accommodate much larger samples and compress them to ultrahigh pressures. [continued on page 10]