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 (HV EM) and high resolution electron microscopy (HREM). The chemical compositio n of their phase boundaries was studied at an atomic level by electron ener gy loss (EELS) fine structure analyses especially at the ionization edges ( ELNES). In specimens annealed in vacuum (T = 1163 K, total pressure about 1 0(-3) Pa), oxygen occurs at the interface only, bound as MgO. However, if t he samples are annealed in oxygen (T = 673 K, partial pressure 10(5) Pa), t he 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 a nd silver consists of oxygen atoms bound partly as MgO and as Ag2O. This se gregation of excess oxygen at the Ag/MgO interfaces fits into the framework of a general structural model, which should be applicable to many metal/ox ide phase boundaries. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.