Nov. 7, 2014
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Shifts in states of matter: It's complicated

Anne M. Stark, stark8 [at], (925) 422-9799

The process of phase changes -- those transitions between states of matter -- is more complex than previously thought.

A team of Lawrence Livermore researchers and colleagues has found that we may need to rethink one of science’s building blocks and illustrate how a proper theoretical description of transitions, so mundane and present in our daily life, has remained unclear.

The researchers examined the way that a phase change, specifically the melting of a solid, occurs at a microscopic level and discovered that the transition is far more involved than earlier models had accounted for.

The discovery is a big step toward a better understanding of melting of complicated systems like ice, or materials used in phase change memory devices.

"This research shows that phase changes can follow multiple pathways, and the mechanistic details are highly sensitive to the physical temperature, which is counter to what is generally known," explains Amit Samanta, a postdoctoral researcher at Lawrence Livermore National Laboratory and the lead author of the study.

The study, which appears in the Nov. 7 edition of the journal Science, also included Mark Tuckerman, a professor of chemistry and applied mathematics at New York University; Tang Qing Yu, a postdoctoral researcher at New York University; and Weinan E, a professor in the Department of Mathematics and the Program in Applied and Computational Mathematics at Princeton University.

This study modeled melting by tracing copper and aluminum metals from a solid to a liquid state using advanced computer models and algorithms. The findings reveal that a phase transition can occur via multiple and competing pathways corresponding to different melting mechanisms. The transitions involve at least two steps, one of which involves formation of a metastable liquid nucleus -- a result that is contrary to classical theories of phase transitions.

The study shows that defects play an important role in melting but their concentration remains surprisingly small -- a conclusion that has remained unclear until now. Further, it is the diffusion of these small defects that play an important role in the formation of the metastable liquid nucleus. These details defy the speculation that melting is correlated with the formation of a large number of point defects or line defects. The study also found that one of the first theoretical descriptions of melting by Frederick Lindemann in 1910 is surprisingly valid at the limit of superheating, a temperature at which the solid basin vanishes, but not at the equilibrium melting point.

Princeton University’s E said that with the advent of new algorithms, the ambiguity in similar classic problems can be resolved.

"Phase transitions have always been something of a mystery because they represent such a dramatic change in the state of matter," Tuckerman, a co-author of the study, said. "When a system changes from solid to liquid, the properties change substantially." He adds that this research helps to fill in existing gaps in basic scientific understanding.

This work is supported by the Department of Energy, the National Science Foundation and the National Natural Science Foundation of China.