Solute carbon and carbon segregation in magnesium oxide single crystals — a secondary ion mass spectrometry study

Abstract

If carbon is to be analyzed by secondary ion mass spectroscopy (SIMS) in an oxide such as MgO, one has to know how the carbon is incorporated in the oxide host structure, before a successful experiment can be planned. If the carbon impurities derive from dissolved CO₂ component which form a solid solution while the crystal grew from a melt in equilibrium with CO₂, upon cooling, the solid solution becomes supersaturated with respect to the volatile CO₂ component. This creates a thermodynamic driving force for exsolution leading to carbon segregation towards the surface. At the surface rapid degassing occurs in vacuum, enhanced by ion bombardment and electron irradiation. Using freshly cleaved synthetic MgO single crystals it can be shown by SIMS (i) that contamination during short exposure to air and during evacuation remains slight, (ii) that rapid surface/subsurface segregation of solute carbon seems to compete with rapid degassing so that, while no extended segregation profile builds up, the carbon concentration in the bulk beneath the surface decreases to a constant level, (iii) that electron irradiation speeds up degassing, (iv) that heating speeds up carbon diffusion, hence its segregation from the bulk, and (v) that Ar⁺ ion sputtering for the purpose of removing possible contaminants reduces the driving force for carbon surface segregation to the point that no segregation profile can be observed. By placing freshly cleaved MgO crystals under isotopically 99 percent pure ¹³CO₂ for various periods of time subsequent SIMS analysis reveals extended ¹²C profiles, probably about 1 μm wide, which can only have formed by ¹²C segregation from the bulk. These results confirm earlier reports that solute carbon exists as mobile impurity in synthetic MgO and natural olivine, probably due to dissolved CO₂ component.