Carbon in oxides and silicates — dissolution versus exsolution

Abstract

Carbon seems to be an ubiquitous impurity in high-melting oxides and silicates which have been exposed to CO/CO 2 during crystal growth. The study of synthetic MgO crystals has provided insight into the dissolution mechanism of CO/CO 2 and the diffusion mechanism of solute carbon through a dense oxide matrix. Treating the dissolution of CO 2 in MgO as a solid solution between the majority divalent oxide and a minority tetravalent oxide, two Mg 2+ vacancies are introduced for each CO 2 molecule dissolved. Carbon, sitting off-center in one cation vacancy, forms an anion complex, CO 2- 2. This complex corresponds formally to a dissolved CO molecule: CO+ O 2= CO 2- 2. The second cation vacancy is chargewise compensated by two O - undergoing spin pairing to give a peroxy anion, O 2- 2, which formally corresponds to a dissolved oxygen atom: O+ O 2-= O 2- 2. Chemically the dissolution of CO 2 can be described as redox process producing reduced and oxidized solute species, e.g. CO 2- 2 and O 2- 2. When the solute carbon passes from the cation vacancy site to interstitial sites, the CO 2- 2 anion complex dissociates into CO - and O -. While one O - remains with the cation vacancy forming a V - center, the C atom diffuses with the other O - state, e.g. a positive hole or defect electron on the O 2- sublattice. Because an O - is much smaller than an O 2- the local lattice contraction associated with the CO - lowers the activation energy barrier for the C diffusion through the dense MgO matrix. This makes C a very mobile solute species, capable of diffusing already at low temperatures when cation vacancy diffusion is not yet activated. At the same time the C diffusion is coupled to the diffusion of an electric charge. Therefore, when the MgO-CO 2 solid solution turns supersaturated during cooling, C segregation builds up a space charge layer near the surface. Its bias counteracts further segregation. This leads to the prediction that the exsolution of the MgO-CO 2 solid solution (e.g. degassing) may be controlled by space charges rather than by the carbon mobility. Experimental support is derived from SIMS measurements. However, if C-C bonds can form at the surface, CO -+ CO -= C 2 O -+ O -, the positive holes, e.g. O - states, may retrodiffuse to the bulk. This reduces the space charge at the surface, while reconverting V - centers in the bulk into peroxy anions which compensate the extrinsic cation vacancies introduced initially. If the C-C polymerization goes on, C n O -=C n+1O -+O -, with n å 1, the exsolution can procede and the solu te CO 2 component eventually splits into graphitic carbon which precipitates at the surface and solute oxygen which remains in the bulk in the form of peroxy defects.