Transmission electron microscopy study of the reaction of Sn^4- ₉ zintl ions with single crystal Au films

The solid reaction products resulting from the interaction of solutions of Sn 4^- 9 Zintl ions with [001] oriented 20 nm thick single crystal Au foils have been analyzed by various transmission electron microscopy techniques. It is found that hexagonal AuSn and free β-tin are the principal reaction products, with evidence of trace amounts of other crystalline phases tentatively identified as AuSn₂, AuSn₄ and K₂Sn₃O₇, in heavily exposed samples. A significant amount of unreacted Au remains even after long exposure to saturated solutions, suggesting that the initial reaction products tend to inhibit further reaction with the Au foil. With exposure to increasing Sn 4^- 9 concentration the AuSn, however, does not appear to cover the Au as a uniform passivating layer but instead, in some areas, grows as fibers that can penetrate the Au foil. There is a preferred growth orientation such that the AuSn [0001] axis always lies within the plane defined by the Au [001] and [110] axes, and such that the AuSn [12̄10] direction remains parallel to the Au [11̄0] direction, within the plane of the foil. This gives rise to selected area diffraction patterns which are a mixture of sharp spots and rings, characteristic of strong textured-fiber epitaxial growth.