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Published on Nov 4, 2011
Most materials expand when heated, but a few contract. Caltech engineers have figured out how one of these curious materials, scandium trifluoride (ScF3), does the trick—a finding, they say, that will lead to a deeper understanding of all kinds of materials.
Heat causes the atoms in the material to vibrate, as seen in this simulation. Because of the linear arrangement of the fluorine atoms (green) bonded with two scandium atoms on each side (yellow), the fluorine vibrates more in directions perpendicular to the bonds. With every shake, the fluorine pulls the scandium atoms toward each other. Although you can't see the contraction of the material here, you can discern the unique vibrations of the fluorine atoms.
The researchers, led by graduate student Chen Li, published their results online on October 24 in Physical Review Letters (PRL). The other authors of the PRL paper, "The structural relationship between negative thermal expansion and quartic anharmonicity of cubic ScF3," are Brent Fultz, professor of materials science and applied physics; former Caltech postdoctoral scholars Xiaoli Tang and J. Brandon Keith; Caltech graduate students Jorge Muñoz and Sally Tracy; and Doug Abernathy of ORNL.