Order in nanometer thick intergranular films at Au–sapphire interfaces

In recent years extensive studies on interfaces have shown that ∼1 nm thick intergranular films (IGF) exist at interfaces in different material systems, and that IGF can significantly affect the materials’ properties. However, there is great deal of uncertainty whether such films are amorphous or partially ordered. In this study specimens were prepared from Au particles that were equilibrated on sapphire substrates in the presence of anorthite glass, leading to the formation of 1.2 nm thick IGF at the Au–sapphire interfaces. Site-specific cross-section samples were characterized using quantitative high resolution transmission electron microscopy to study the atomistic structure of the films. Order was observed in the 1.2 nm thick films adjacent to the sapphire crystal in the form of “Ca cages”, experimentally demonstrating that ordering is an intrinsic part of IGF, as predicted from molecular dynamics and diffuse interface theory.

► Au particles were equilibrated on (0 0 0 1) sapphire in the presence of anorthite.
► 1.2 nm thick equilibrium films (complexions) were formed at the Au–sapphire interfaces.
► Quantitative HRTEM was used to study the atomistic structure of the films.
► Structural order was observed in the 1.2 nm thick films adjacent to the sapphire crystal.
► This demonstrates that ordering is an intrinsic part of equilibrium intergranular films.