In research published in “Proceedings of the National Academy of Sciences,” scientists at Lawrence Livermore National Laboratory (LLNL) describe how liquid metals crystalize under immense pressure.
In 1879, Nobel-prize winning chemist Friedrich Wilhelm Ostwald discovered that liquids often first freeze into temporary, unstable structures before changing into their final, most stable equilibrium phase. This so-call “Ostwald step rule” has been a fundamental mechanism for the study and synthesis of new materials and is a textbook principle that all physicists learn in school.
But in their study, LLNL researchers Babak Sadigh, Luis Zepeda-Ruiz and Jon Belof describe a new mechanism of solidification in copper that provides a more detailed analysis of the Ostwald’s step rule and alters the fundamental understanding of nucleation under extreme pressure. They found that not only do metals first crystalized into an unstable or non-equilibrium phase, but that this phase can be stabilized by temperature.
A precise understanding of how “Oswalt stepping” occurs at the elementary, or atomistic scale, has long been a scientific mystery for liquids compressed to high density, such as the solidification of molten iron that is believed to occur at the center of the Earth.
But using simulations run on a supercomputer, the LLNL researchers were able to predict how crystalizing metals will react in dynamic experiments.
“Using modern simulation methods, we have tried to provide a framework for predicting how process conditions can be manipulated to design completely new materials using both a modern laser platform as well as classical heating in furnaces,” according to lead author Babak Sadigh.
Jon Belof, principal investigator for phase transition kinetics and program leader for the Equation of State program at LLNL, said the results of the simulations were surprising.
“It has forced us to rethink the basis for phase stability at high pressure,” Belof said. “Under these extreme conditions, such as in the center of Earth, the thermodynamic phase diagram sort of goes out the window in terms of predictive capability and one needs to turn to kinetic phase maps.”
The research is funded by the National Nuclear Security Administration.