By Josh Perry, Editor
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Researchers at Cornell University (Ithaca, N.Y.) studied the perovskite lead titanate (PbTiO₃) and determined that the commonly-held rules for understanding materials with negative thermal expansion (NTE), which expand when cooled, do not apply to all and that a new map for these materials was needed.

Cornell researchers determined that the assumptions about materials with NTE were wrong and a new map for determining these materials was needed. (Wikimedia Commons)

According to a report from the university, the researchers used theory and computer simulations to analyze NTE in lead titanate, which undergoes a phase transition at 908°F and has volumetric NTE down to room temperature, and found two factors that were considered necessary for NTE were not necessary.

“Their work highlights the important role played by two material properties that are usually ignored: the behavior of the electrons and the material’s elastic properties (the ability to stretch or compress a material along different axes),” the report explained.

By creating a new map for NTE materials, the researchers are hoping to make the design of other NTE materials easier.

The research was recently published in Physical Review Letters. The abstract read:

“We use first-principles theory to show that the ingredients assumed to be essential to the occurrence of negative thermal expansion (NTE)—rigid unit phonon modes with negative Grüneisen parameters—are neither sufficient nor necessary for a material to undergo NTE.

“Instead, we find that NTE in PbTiO3involves a delicate interplay between the phonon properties of a material (Grüneisen parameters) and its anisotropic elasticity.

“These unique insights open new avenues in our fundamental understanding of the thermal properties of materials and in the search for NTE in new materials classes.”