Physicist Seeks Ways To Optimize Shape-Changing Crystals

FAYETTEVILLE, Ark. - When a doctor puts a small device on pregnant woman’s belly, an ultrasound image of an unborn baby appears on an ultrasound screen. When a submarine sends out sonar signals a device on the nose catches the reflected signals and sends an image to an inside screen. Working with models of the materials responsible for those pictures, a University of Arkansas physicist has found the key to optimizing their efficiency.

Laurent Bellaiche, assistant professor of physics, A. Garcia of the University del Pais Vasco, Bilbao, Spain, and David Vanderbilt of Rutgers University recently presented their work at the American Physical Society meeting in Minneapolis.

The materials that create ultrasound and sonar images, called piezoelectric compounds, convert mechanical energy to electricity and back again. A new family of these compounds, discovered in 1997, has an enhanced effect of more than 10 times the original materials, which could lead to drastic improvements in ultrasound and sonar resolution. However, little is known about the mechanism for this magnified effect.

Bellaiche and his colleagues study the underlying reasons for the enhancement and have developed a mathematical tool that explains the phenomenon.

"Now that we understand the effect, we can optimize it," he said.

The atoms in the newer family of piezoelectric compounds have different valences, or abilities to accept electrons, than the atoms from the older group. Because of this, playing with the atomic ordering will lead to an optimization of the mechanical-electrical effect, Bellaiche said.

The mathematical model also reproduces the different structural phases observed in piezoelectric compounds. At high temperatures, the compounds take on a cubic shape and lose their piezoelectric properties, while at lower temperatures they lose this symmetry and form tetragonal and rhombohedral shapes and produce the mechanical-electrical effect.

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