Random Walk
Staying Firm Under Pressure
Back in the day, before the meter was defined in terms of the speed of light, it was set as 1/10,000,000 of the distance between the equator and either pole along the meridian that passes through Paris. Rather than sending out surveyors whenever someone needed to know the exact length of a meter, the French Academy of Sciences created a meter-long metal bar known as the prototype. Made out of an unusually thermally stable but very expensive platinum-iridium alloy, the prototype was kept in a vault at the International Bureau of Weights and Measures in Sèvres, France, near Paris. Replicas of the prototype would be made and distributed as needed. The replicas were created using the same alloy, making them unaffordable to most people. Using cheaper materials was undesirable due to thermal expansion—until 1896, when Swiss scientist Charles Édouard Guillaume, head of the bureau, discovered an inexpensive iron-nickel alloy that would not expand when heated. Since then, several other "Invar" materials (so named because of their temperature-invariant properties) have been discovered. Though their internal mechanics remain controversial, modern Invar alloys find applications in many everyday items, such as toasters, computers, watches, and light bulbs.
Now Caltech grad student Michael Winterrose, Professor of Materials Science and Applied Physics Brent Fultz, and their colleagues have found a new way to induce Invar behavior in materials. By placing an iron-palladium alloy under high pressure, they were able to change it from an alloy that displayed no Invar behaviors to one that undoubtedly did.
Winterrose and Fultz were studying alloys composed of one part iron and three parts nickel, palladium, or platinum. Though nickel and iron atoms are nearly the same size, palladium and platinum are much larger. Winterrose and Fultz had planned to investigate the effects of high pressure on materials made of atoms of mismatched sizes, versus alloys of similarly sized atoms, hoping to discover interesting volume effects.
Winterrose and Fultz would place tiny samples of their alloys between two diamonds in what is called a diamond anvil cell. Tightening six screws forces the diamonds together, generating pressures of up to 33 gigapascals (GPa), or more than 300,000 times atmospheric pressure. With temperature kept constant, if pressure is increased, most materials contract at a relatively constant rate.
For the palladium-iron sample, however, that was not the case. At around 10 GPa, the Pd3Fe sample began to compress more easily than before, and at around 15 GPa, the material became stiffer than it had been originally. This strange pressure-volume curve was baffling—that is, until Winterrose recalled that similar curves appear in a few other iron alloys. However, those curves were associated with Invar materials, which have unique magnetic properties that cancel out thermal expansion—properties that Pd3Fe does not normally exhibit.
A computer simulation suggested that the high-pressure stiffening of the material might be due to a magnetic transition. "Perhaps the best early hint was found in the calculations of electron energies in Pd3Fe. For high pressures, the spectrum of electron energies showed the fingerprints of an instability, where the magnetism was about to collapse, and a different set of electron levels would then become occupied," recalls Fultz.
A rendering of electron density surfaces surrounding the iron (yellow) and palladium (blue) atomic cores in Pd3Fe, based on calculations from first-principles quantum mechanical simulations. At a pressure of 12 GPa, the electron density begins to migrate toward the iron atoms, leading to Invar behavior.
Armed with their sample, Winterrose, Fultz, and company went to the Advanced Photon Source at the Argonne National Laboratory. There, they used a technique called nuclear forward scattering that allowed them to excite the magnetic states of their material, confirming that such a transition was indeed taking place—in fact, at the same pressures where the volume collapse occurred. As the electron configuration changed, the energy structure became closer to that of the traditional Invar material Fe3Pd. "In this way," Winterrose says, "it's like alchemy. We've coaxed one material into behaving like another."
Finally convinced that they had an Invar material on their hands, Winterrose and Fultz performed one last test—they heated their troublesome alloy while it was being pressurized. And sure enough, they found that it did not expand at temperatures up to 250 degrees Celsius.
"Now that we've found an electronic structure change associated with Invar behavior, perhaps we will be able to find other means to cause similar changes and so create new Invar materials at lower pressures," Winterrose says. "This also demonstrates the possibility of manipulating the electronic properties of matter using simple mechanical force, and offers insight into the materials existing at high pressures that make up a large part of the matter in our solar system."
A paper describing their work was published in the June 12 issue of Physical Review Letters. In addition to Winterrose and Fultz, the coauthors are Matthew S. Lucas (MS '05, PhD '09), grad students Alan F. Yue, Lisa Mauger, and Jorge Muñoz (MS '09), and visiting scientist Itzhak Halevy, from Caltech; Jingzhu Hu, from the University of Chicago; and Michael Lerche, from the Carnegie Institution for Science.
The work was supported by the Carnegie–Department of Energy (DOE) Alliance Center, funded by the DOE through the Stewardship Sciences Academic Alliance of the National Nuclear Security Administration; by the DOE's Office of Science, Office of Basic Energy Sciences; by the National Science Foundation and its Consortium for Materials Properties Research in Earth Sciences (COMPRES); and by the W. M. Keck Foundation. —AL
