Computational theory reveals an unusual quality in a pure compound.
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“If you look at something closely that is thought to be well understood, you often find something new and exciting,” says Ronald Cohen, speaking of his latest work with lead titanate. Researchers thought that this simple compound could be “easily understood” as early as the 1950s. But Cohen, of the Carnegie Institution of Washington Geophysical Laboratory, and his colleague Zhigang Wu, revealed an unexpected property, using a new theory developed by computational physicists to calculate the properties of piezoelectrics — substances that convert electrical energy to mechanical energy and vice versa.
When they applied the theory in the context of extreme pressures, Wu and Cohen discovered that lead titanate would undergo a set of unexpected phase transitions never before seen in a pure compound but usually associated with more complex, and commonly used, piezoelectric materials. These structural changes — and the electromechanical responses that accompany them — render such materials useful for a range of applications, from medical ultrasound to sonar.
Wu and Cohen's results suggested that simple compounds could be developed for similar applications. The computed piezoelectric properties of pure lead titanate under pressure were greater than those of any known material.
After celebrating their theoretical discovery, the Geophysical Lab team decided to put the theory to experimental test. The first thing the researchers had to do was create the conditions in which lead titanate was expected to take on the properties in question and find ways to measure these properties. “To do both together is extremely difficult,” says Cohen. The team first tried using Raman spectroscopy to measure atomic vibrations at high pressures and low temperature (10 kelvin). “Those experiments showed interesting behaviour for the compound, but we didn't know exactly what it meant,” says Cohen.
They turned to X-ray diffraction, using the Advanced Photon Sources at the Argonne National Laboratory in Illinois, again under high pressures and at cryogenic temperatures. To obtain useful data they had to perform high-resolution diffraction experiments, which take hours rather than minutes or seconds. “It's difficult to get that amount of synchrotron time, and it's also hard to maintain the required pressure and temperature for so long,” says Cohen.
But the authors' perseverance paid off. They eventually obtained a high-resolution image of lead titanate undergoing a morphological metamorphosis that Cohen describes as “akin to making a cube switch from sitting on a face to balancing on a corner” (see page 545). This type of structural change is associated with a large electromechanical response.
The team decided to take the work a step further and used this information to design a compound that would have similar electromechanical properties to lead titanate but at ambient temperature and pressure. The next challenge is to actually make this material and confirm its properties.
“If, as I hope, we succeed in developing useful new materials by this route, we will have shown the utility of a materials-by-design approach for the next generation of technological materials,” Cohen says.
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Ronald Cohen. Nature 451, xi (2008). https://doi.org/10.1038/7178xia
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DOI: https://doi.org/10.1038/7178xia