Surface and bulk phases in substituted cobalt oxide spinels

Samples of cobalt oxide spinel doped with a range of cations (Li, Ni, Zn, Cu, Al, Ga, Mn, Ti, Sn, Ge) have been prepared and examined by X-ray diffraction and X-ray photoelectron spectroscopy. Surface enrichment in the dopant species is frequently observed. Determination of the ratio of the two cobalt oxidation states in the surface layers is found to be useful in distinguishing between solid solution behaviour and the nucleation of surface phases. This ratio may also be helpful in characterising the preferred occupancy of octahedral and tetrahedral sites in the near surface regions.