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In a serendipitous discovery, an electrically conductive compound of ytterbium, gallium, and germanium has been shown to maintain its room-temperature volume when heated between 100 and 400 K. The compound, YbGaGe, was prepared by chemistry professor Mercouri G. Kanatzidis, solid-state chemistry graduate student James R. Salvador, and coworkers at Michigan State University [Nature, 425, 702 (2003)]. Zero-thermal-expansion (ZTE) materials prevent or reduce the strain or internal stresses that can occur in systems subjected to large temperature fluctuations, such as in space applications and thermomechanical actuators, the authors note. Potential applications of electrically conducting ZTE materials include multilayered printed circuits, which can suffer from thermal expansion problems, according to Kanatzidis. "Conductive materials also reflect radiation," he adds. "They can be used as specialized mirrors, for example, in spectroscopic instrumentation used in astronomy where changes in mirror performance cannot be tolerated." Most solids expand on heating (positive thermal expansion). The few materials that expand on cooling are known as negative-thermal-expansion (NTE) materials. They include a number of oxides. To date, the rare examples of ZTE materials are all composites rather than pure materials, and they are all insulators. They are prepared by combining NTE materials with positive-thermal-expansion materials. The composites exhibit ZTE over narrow temperature ranges of 5 to 10 °C. The range for YbGaGe is an impressive 300 °C, Kanatzidis points out. "The YbGaGe composition can be tuned to become not only an NTE material but also a ZTE material," he tells C&EN. "In fact, zero volumetric thermal expansion is achieved in most cases. So YbGaGe does not have to be mixed with positive-expansion materials to achieve the ZTE property. "The discovery of YbGaGe was inadvertent in that we were interested in preparing new compounds in the Yb-Ga-Ge system using liquid Ga as the solvent," he continues. "When we isolated YbGaGe, we noticed it had unusual magnetic properties. As the temperature dropped, the magnetic moment of Yb dropped substantially, suggesting a Yb3+ to Yb2+ transition." MIXED VALENCY Electron transfer occurs in the Yb(1) layer, resulting in oxidation of some Yb2+ ions to Yb3+. The Yb(2) layer consists only of Yb2+ ions. Yb = yellow, Ga = red, and Ge = blue. As Yb2+ is larger than Yb3+, the researchers suspected that the compound might exhibit anomalous thermal expansion behavior. They confirmed this behavior by determining the unit-cell dimensions of the compound from crystallographic data collected over a range of temperatures. "The unit-cell dimensions change so that two of them expand when the material cools, whereas the third contracts, leading to constant volume," Kanatzidis explains. The researchers propose that an electronic mechanism rather than a geometric configuration change accounts for the anomalous behavior. "We speculate that electrons delocalized over a band of orbitals leave this band and become localized on a specific atom as the temperature drops," Kanatzidis says. "For this to happen, the electron-accepting atom--Yb, for example--has to be capable of mixed valency. As the atom becomes reduced, it expands. The part of the structure that loses the electron does not seem to contract as much because the electron is delocalized over a large area." The net result is zero thermal expansion. "The discovery permits a more rational search for NTE and ZTE materials among semiconductors and intermetallics, which was not thought of before," Kanatzidis suggests. "Perhaps other systems can now be devised that take advantage of valence transition. It is a fresh approach to NTE and ZTE." |
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