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BNL Research Reactor Uncovers a New Compound's Surprising (and Useful) Properties

Upton, NY -- Using a research reactor at the U.S. Department of Energy's Brookhaven National Laboratory (BNL), scientists have discovered that a new zirconium compound defies all expectations by shrinking when it's heated.

As reported by researchers from Oregon State University and BNL in the April 5 issue of Science, the compound zirconium tungstate -- made of zirconium, tungsten and oxygen -- behaves like no other known material. And its unique properties, the scientists say, could be put to good use by industry.

Solid objects usually expand when they're heated and shrink when they're cooled, as the bonds between the atoms in their molecules lengthen and shorten. Some ceramics, such as those used in some bakeware, are made of molecules that shrink in one direction but expand in others when heated -- a property that keeps the pans from breaking easily.

But zirconium tungstate is completely different. Instead of expanding when it was heated to temperatures ranging from near absolute zero (- 459.4 degrees Fahrenheit) to 1,050 Kelvins (1430 degrees F), it underwent what scientists call negative thermal expansion. In other words, it shrank. But unlike the bakeware, it shrank uniformly in all directions.

This behavior, the researchers say, makes the compound more than just an oddity. Its unusual traits may prove useful for the manufacturers of everything from circuit boards to telescopes, who could incorporate it into their products to prevent the warping and cracking that often occurs when materials are exposed to rapid changes in temperature. The shrinking zirconium tungstate would counteract the expansion of conventional materials, creating products that neither contract nor expand when heated or cooled.

The research that uncovered zirconium tungstate's structural properties was performed at BNL's High Flux Beam Reactor (HFBR), a 30-megawatt research reactor that welcomes university and industry scientists as guest researchers.

As part of its ongoing research on the zirconium compound, the Oregon State team, led by chemist Arthur Sleight, sent its samples of the material to the HFBR for examination. The team worked with BNL physicist Tom Vogt, who built and operates a powerful reactor-based instrument that can sort out atomic structures.

Vogt offered a possible explanation for the compound's behavior. "It is thought that the oxygen atoms within the compound vibrate more strongly at increased temperatures, pulling the zirconium and tungsten atoms together," he said.

Called a neutron powder diffractometer, the instrument makes use of subatomic particles called neutrons, which are produced in the reactor¹s core and channeled to the diffractometer through a sealed horizontal tube called a beam line. The HFBR gives scientists a constant flow of neutrons for their experiments, unlike a nuclear power reactor, which operates at a hundred times the HFBR¹s power level but is designed to give off heat that can be used to produce electricity.

The powder diffractometer, positioned at the end of an HFBR beam line, allows neutrons to pass through a small sample of the material being studied. When neutrons hit the individual atoms within the sample, they scatter, or diffract, onto the instrument¹s detector. The patterns made by neutrons in the detector can then be analyzed by a computer to give a three-dimensional view of the material's internal atomic structure.

At the BNL reactor, the material was heated to different temperatures and placed in the diffractometer for examination. The resulting images gave exact locations for each zirconium, tungsten and oxygen atom inside the sample. By careful analysis of the atoms' location shifts over the entire temperature range, the surprising finding came to light.

The zirconium tungstate research was one experiment out of more than 100 performed at the HFBR's powder diffractometer annually. The BNL instrument produces higher-resolution images than any other instrument of its kind, and was able to sort out the atomic structure of zirconium tungstate down to a precision of the 0.004 angstroms, the distance between individual zirconium and oxygen atoms in the compound. An angstrom is a hundred millionth of a centimeter. The instrument was built in 1993 through funding from the Department of Energy¹s Office of Basic Energy Sciences.

Brookhaven National Laboratory carries out basic and applied research in the physical, biomedical and environmental sciences and in selected energy technologies. Brookhaven is operated by Associated Universities, Inc., a nonprofit research management organization, under contract with the U.S. Department of Energy.

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