The Phase Behavior of Interfaces

Grain boundaries between crystals, which can control materials properties, can interconvert between well-defined equilibrium structures. Common metal or ceramic objects are made up of many bonded micrometer-scale single crystals, or grains. Their interfaces, called grain boundaries, play a decisive role in determining the properties and processing of almost all engineering materials. Because of their structural and chemical complexities, the description of grain boundaries has lacked a satisfactory conceptual framework, and grain boundaries tend to be viewed as disordered regions with kinetically trapped structures. Recent work has shown that grain boundaries can be described as interface-stabilized phases (also called interphases or complexions) that are chemically and structurally distinct from any bulk phases (1–4). On page 206 of this issue, Baram et al. (5) extend our understanding of grain boundaries by examining interface structures that form between a gold particle in contact with metal oxide surfaces. They show that these nanoscale interface structures are equilibrium phases that obey thermodynamic rules analogous to those for bulk phases.