Atomic carbon in magnesium oxide Part VIII: General discussion and outlook

The presence of atomic carbon in MgO and the experimentally well documented fact that an internal equilibrium exists between defect-bound OH^? and H + O^? lead to a very complex pattern of defect equilibria and defect reactions in the bulk and at the surface of the MgO crystals. The most important extrinsic bulk defects are the bare and the OH-compensated cation vacancies, V″_$\text{Mg}$]″, |OH^· V″_$\text{Mg}$|′, and OH^· V″_$\text{Mg}$ HO^·]^$\text{x}$ of which the latter yields O^· (H_$\text{2}$)″_$\text{Mg}$ O^·]^$\text{x}$, interstitial hydrogen molecules (H_$\text{2}$)^$\text{x}$, interstitial C atoms C^x_i, which may enter the available cation vacancies, and the positive hole, O^·]^?. Reactions between C atoms entering the O V″ O and O^· V″_$\text{Mg}$|′ at the surface lead to the formation of CO_$\text{2}$ and CO molecules and to the formation of double F and single F centers respectively. The free electrons in the F centers are held responsible for the formation of Mg-organic commpounds. Reaction between the C atoms and interstitial H₂ molecules lead to hydrocarbon formation. The strain fields associated with the interstitial C atoms and the positive holes provide the driving force for segregation of the carbon into the elastically relaxed subsurface zone and for fluctuating reaction kinetics.