The geometrical and chemical structure of the CoAl(111) surface is investigated by quantitative low-energy electron diffraction and calculations applying density functional theory. The stacking sequence of the top four atomic planes is Al-Co-Co-Co, followed below by the usual alternating B2 stacking. The topmost layers thus form a unit cell of the well-known bcc-based D0(3) crystal structure [the A(3)B superlattice of bcc(111) atomic planes], although the bulk phase diagram of CoAl shows no D0(3) phase. Its occurrence and stability at the surface is due to a slight Co excess of the nominally stoichiometric sample, equivalent to the presence of Co antisite defects in the bulk. These defects are enriched in undercoordinated near-surface sites of the Al sublattice, which lowers the total energy because more At atoms can then reside in fully coordinated bulk At sites. However, all three topmost layers are undercoordinated, and the segregation of Co antisite defects competes with a general trend towards a termination of the surface by Al. In the balance, the third layer is the preferred plane for Co antisite defects.

For the example of the B2 CoAl(100) surface, we demonstrate that even slight deviations from an ordered alloy's ideal stoichiometry in a subsurface region or in the bulk can drastically affect its surface composition. By experimental surface analysis and first-principles calculations, we show that Co antisite atoms segregate to the very surface, driven by the same strong interactions which enforce order in the bulk. Our findings are consistent with the lack of antisite segregation we found earlier for the much weaker ordering FeAl(100), and resolve contradictory reports for NiAl(100).

A novel titanium precursor was used for TiO2 deposition in low pressure chemical vapor deposition reactor. The deposition kinetic was experimentally studied and used in a detailed three-dimensional reactor model. The model was validated by comparison with the experimentally observed growth rate distribution as a function of the growth temperatures.