Effects of O-2 gas on the size and structure of Cr clusters formed by plasma-gas-condensation

Cr clusters have been produced by a plasma-gas-condensation type cluster de position apparatus, and studied using a time-of-flight mass spectrometer an d a transmission electron microscope. The Cr clusters formed in high pressu re inert (Ar and/or lie) gas atmosphere are of an A15-type structure. When an O-2. Ps is mixed with the inert gases in the source (sputtering) chamber , a bcc phase is formed together with Cr2O3. The O-2 gas introduction leads to an increase in the gas temperature of the source chamber probably due t o release of the formation enthalpy of the oxide. The A15 phase is annealed by such excess heat and becomes the equilibrium bee phase. The sizes of be e clusters are smaller than those of the A15-clusters, probably due to the heterogeneous nucleation promoted by the oxide formation.