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

A quantitative study of space charge solute segregation at grain boundaries in TiO2 is conducted, using a new STEM method for the measurement of aliovalent solute accumulation. It is shown that the electrostatic potential at grain boundaries can be varied in sign and magnitude with doping, oxygen pressure, and temperature, and that the isoelectric point lies in slightly donor-doped compositions for samples annealed in air. The experimental results closely fit the space charge model in Part I. Space charge solute segregation is found even in defect regimes of high electron concentration. Approximately one in ten grain boundaries are ''special'' in exhibiting no detectable segregation; in one such instance a twin boundary is identified. Among boundaries with significant amounts of segregation, clear differences in potential also exist. From the potential determined in acceptor- and donor-doped compositions, the Frenkel energy (assumed to be lower than the Schottky energy in TiO2) can be separated into its individual terms. An average value for the titanium vacancy formation energy of g(V)Ti = 2.4 eV and an upper limit to the titanium interstitial formation energy of g(Ti)i = 2.6 eV are obtained.