Evidence of oxygen segregation at Ag/MgO interfaces

The distribution and microstructure of MgO precipitates formed by internal oxidation of Ag/3 at.% Mg alloys have been investigated by high voltage (HVEM) and high resolution electron microscopy (HREM). The chemical composition of their phase boundaries was studied at an atomic level by electron energy loss (EELS) fine structure analyses especially at the ionization edges (ELNES). In specimens annealed in vacuum (T = 1163 K, total pressure about 10(-3) Pa), oxygen occurs at the interface only, bound as MgO. However, if the samples are annealed in oxygen (T = 673 K, partial pressure 10(5) Pa), the spatially resolved fine structure of the energy loss O-K edge reveals an additional interface bonding part, representing an Ag2O-like bonding state. Thus, with high oxygen activities, the interface layer between magnesia and silver consists of oxygen atoms bound partly as MgO and as Ag2O. This segregation of excess oxygen at the Ag/MgO interfaces fits into the framework of a general structural model, which should be applicable to many metal/oxide phase boundaries. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.