PhD Theses - Shyam Vyas
Simulation of Ceria: Bulk and Surface Defects
Atomistic computer simulation techniques, based on the classical pair potential model, have been used to study several inorganic materials: primarily CeO2, but also CaF2 and MgO. Both bulk and surface properties have been investigated.
In bulk ceria defect clusters incorporating indium or cadmium dopants with oxygen vacancies have been modelled. The binding energies show an isolated Cd2+ defect prefers to be in a first nearest neighbour site with respect to an oxygen vacancy; the In3+ ion shows no such preference. When Cd2+ and In3+ bind to Ce3+ ions (small polarons present in CeO2−x), at least one of the ions must be at a second neighbour site with respect to the vacancy. The same is true for small polaron clusters in undoped non-stoichiometric ceria.
The importance of defect-defect interactions has been assessed through the study of the solution of Al2O3 in MgO. Two defect models are examined, trimer and dimer clusters of the Al substitutional ions with the charge compensating V'' species dominate inate. The results agree favourably with the experimental lattice parameter data.
The dependence of the surface structure and crystal morphology on the interionic potential was examined. The resultant morphology is dominated by (111) faces, irrespective of the potential. In addition, studies of the angular dependence of the surface energy have been carried out. These demonstrate that the surface energies lie on a well defined curve. A physical basis for this trend is proposed.
Initial studies of non-stoichiometric surfaces of CeO2 have been carried out. The configurations of neutral polaron clusters on a number of important faces are examined. The results suggest that for the (200) face the vacancy prefers to be at the surface; for the (331) face the opposite is true. On the remaining faces, the defects rearrange such that they reduce the defect dipole in the Z-direction.
Finally, molecular dynamics techniques have been used to study two CaF2 nanoclusters. The results are compared with those of static surface calculations. In addition, studies of melting and solidification are carried out on the smaller cluster. In only one case was the cooling rate sufficiently slow to form an ordered structure.