SPACE-CHARGE SEGREGATION AT GRAIN-BOUNDARIES IN TITANIUM-DIOXIDE .2. MODEL EXPERIMENTS

A quantitative study of space charge solute segregation at grain bound aries in TiO2 is conducted, using a new STEM method for the measuremen t of aliovalent solute accumulation. It is shown that the electrostati c potential at grain boundaries can be varied in sign and magnitude wi th doping, oxygen pressure, and temperature, and that the isoelectric point lies in slightly donor-doped compositions for samples annealed i n air. The experimental results closely fit the space charge model in Part I. Space charge solute segregation is found even in defect regime s of high electron concentration. Approximately one in ten grain bound aries are ''special'' in exhibiting no detectable segregation; in one such instance a twin boundary is identified. Among boundaries with sig nificant amounts of segregation, clear differences in potential also e xist. From the potential determined in acceptor- and donor-doped compo sitions, the Frenkel energy (assumed to be lower than the Schottky ene rgy in TiO2) can be separated into its individual terms. An average va lue for the titanium vacancy formation energy of g(V)Ti = 2.4 eV and a n upper limit to the titanium interstitial formation energy of g(Ti)i = 2.6 eV are obtained.