Nicole Benedek, Cornel University
Exploring the Crystal Chemical Origins of Negative Thermal Expansion in Inorganic Framework Materials
Monday, November 2
Abstract:
Most materials shrink when cooled but some, such as ice, expand instead. This phenomenon, known as negative thermal expansion, is relatively rare and the mechanisms that give rise to it are only understood in detail in a handful of cases. In inorganic framework materials, such as zeolites and perovskites, there are two ingredients commonly thought to be essential to negative thermal expansion: the existence of low-frequency rigid unit phonon modes (RUMs), in particular those with negative Grüneisen parameters, that is, phonons with frequencies that decrease with decreasing volume. The RUM model of negative thermal expansion successfully accounts for the thermal behavior of a number of materials, including the canonical negative thermal expansion material ZrW2O8. Can this model explain why the tetragonal perovskite PbTiO3 undergoes negative thermal expansion?
We use theory and first-principles calculations to elucidate the microscopic mechanism of negative thermal expansion in ferroelectric perovskite PbTiO3. In non-cubic systems, such as tetragonal PbTiO3, the thermal expansion along a given axis is coupled to multiple Grüneisen parameters through multiple independent elastic constants. We show that few of the modes critical to driving negative thermal expansion in PbTiO3 have negative Grüneisen parameters and that they are not RUMs, even though PbTiO3 contains many RUM-like modes. We then connect the physical mechanism of negative thermal expansion in PbTiO3 to its electronic structure and crystal chemistry, and show that its elastic properties are dominated by the stereochemical activity of the Pb2+ 6s2 6p0 lone electron pair. Our results suggest that PbTiO3 is unique among well-studied inorganic framework negative thermal expansion materials in that it appears to be the only material among this group that exhibits volumetric negative thermal expansion well above room temperature, and with positive Grüneisen parameters along all unique crystallographic axes.
Want to attend?
We aren't advertising our Zoom links, but you're welcome to join! Just email Alycia Alexander (adalexander@carnegiescience.edu) for information on how to attend. If you are a staff member you will receive a Zoom link before each seminar.