The correlation of subsurface oxygen diffusion with variations of silver morphology in the silver-oxygen system

Silver undergoes pronounced morphological changes following high-temperatur e treatment in various gas atmospheres. SEM analysis shows that high-temper ature treatment in oxygen leads to pronounced facetting of the silver surfa ce. In situ XRD shows a strong texturing of the polycrystalline bulk and an expansion of the unit cell resulting from the dissolution of oxygen. Two d istinctly different forms of subsurface oxygen have been identified and the influence of morphological changes on the mechanism leading to their forma tion clarified. The first of these species is O-beta. It is bulk-dissolved oxygen which diffuses via an interstitial mechanism through low-resistance diffusion paths such as grain boundaries and open crystalline planes. Tempe rature programmed desorption spectroscopy analysis reveals that high-temper ature oxygen pretreatment results in the formation of crystalline surfaces exhibiting different oxygen diffusion barriers. At elevated temperatures, p ure thermal reordering dominates over oxygen-induced restructuring of the s ilver bulk and surface. The near-surface region is, therefore, comprised pr imarily of close-packed crystalline planes at elevated temperatures, The ba rrier for interstitialcy diffusion is overcome at temperatures in excess of 923 K, allowing bulk-dissolved, atomic oxygen to (O-beta) segregate into t hese low-indexed planes. This likely occurs via an interstitialcy diffusion mechanism where oxygen substitutes for silver atoms in the lattice. This o xygen species is referred to here as O-gamma, Analysis of ISS depth profili ng of silver foils treated under various conditions shows that the diffusio n coefficient is a positive function of the oxygen concentration. This is a direct result of the oxygen-induced recrystallization of the silver result ing in a lowered barrier to diffusion. In situ XRD shows that oxygen locate d in octahedral holes of silver preferentially diffuses in the [110] direct ion. This requires the anisotropic displacement of silver atoms. The result ing strain culminates in a slight expansion of the silver unit cell and the preferential growth of crystals in the shape of needles. STM imaging of th e surface shows the macroscopic silver facets to be composed of smaller, co lumnar crystallites, thus confirming the validity of the model based on the XRD data. (C) 1999 Academic Press.