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Mysterious material has unusual electrical properties

Scientists at Brookhaven National Laboratory are studying a mysterious material that may lead to significant advances in the miniaturization of electronics

This molecular model shows the arrangement of atoms of calcium (yellow), oxygen (red), copper (blue), and titanium (black, at center of double-sided brown pyramids, or octahedra). The grey box in the upper right-hand-side of the figure represents one unit cell of the material.

August 20—In the July 27, 2001, issue of Science magazine, the scientists describe findings that offer the first clues to explain the material's newly discovered, unusual electrical properties. This work may lead to applications using the material to store electrical charge in high-performance capacitors, and offer insight into how charges behave on the nanoscale-on the order of billionths of a meter.

The material—a perovskite-related oxide containing calcium (Ca), copper (Cu), titanium (Ti), and oxygen (O) in the formula CaCu3Ti4O12—is unusual in that it has an extremely high dielectric constant, a property that determines its ability to become electrically polarized (i.e., separate positive and negative electrical charges). The higher the dielectric constant, the more charge you can store, and the smaller you can make electronic circuits.

In addition, unlike most dielectric materials, this one retains its enormously high dielectric constant over a wide range of temperatures, from 100 to 600 Kelvins (K), or -173 to 327°C, making it ideal for a wide range of applications. Yet the material's dielectric constant drops precipitously—1,000-fold-below 100 K, with no evidence of structural or phase changes in the atoms. Therein lies the mystery.

"Such a large change in the way charge is distributed within the material implies that the atomic structure should change as well," said Christopher Homes, the lead physicist on the Brookhaven study. "It's difficult to imagine how one property can undergo such a large change while the other remains unaffected."

Previously, scientists have looked for hints of changes using x-rays, neutron beams, and other methods—to no avail. But Homes' technique, measuring optical conductivity, or the material's ability to reflect and absorb varying frequencies of infrared light, revealed a number of unusual changes in the way the atomic structure vibrates.

The scientists detected the vibrations by illuminating samples of the substance with varying wavelengths of infrared light at Brookhaven's National Synchrotron Light Source, and measuring which wavelengths were reflected and which were absorbed. The absorbed wavelengths are those that match the atoms' natural vibration frequencies. As the temperature of the substance was cooled below the 100 K mark, the absorbed frequencies—and therefore the vibrations—changed.

"Since the vibrations in a solid depend a great deal on how the charges are distributed, the changes in vibrations suggest that the charges can be rearranged without causing a structural distortion," Homes said. "The fact that we see these changes offers the first real glimpse of why this material has such a large dielectric constant, and the mechanism by which it decreases so dramatically below 100 K."

The scientists speculate that at temperatures above 100 K, pairings of positive and negative electric charges, called dipoles, can flip around quickly, independent of one another. This property and the high concentration, or density, of dipoles within the solid both contribute to the large dielectric constant. If you put the material in an electric field, all the individual dipoles flip into alignment to separate the charges.

But as the material cools, the dipoles "freeze out" in random positions, losing their ability to flip quickly into alignment. This "electronic phase transition" happens in the absence of a structural change. "Additional research will help us understand this effect and the range of ways this material might be used in microelectronics and other fields," Homes said.—by Karen McNulty Walsh



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