Oxidation of scandium by oxygen and water studied by XPS

Reaction of scandium with water at 293 K gives initially the oxide and, above approximately 50 L, also the hydroxide. The ratio of hydroxide to oxide is approximately constant in the later stages of reaction. The reaction of water with scandium is slower and less extensive than with the lanthanides. At low exposures of water at 80 K only the hydroxide and chemisorbed water are produced, again in the contrast to the lanthanides. Oxidation with oxygen is complex. For small adsorptions of oxygen, the O 1s BE does not agree with the value for the bulk oxide, whereas for a large step adsorption, they are in good agreement. The overlayer depths calculated from the metal and from the oxygen signals also differ for stepwise additions of oxygen. The results are tentatively interpreted as dissolution of the oxygen in the scandium, with the oxygen concentration below the value required for nucleation of oxide. (Auth.)