Undoped TiO2 exhibited deterioration in microwave dielectric loss as it reached its

maximum density due to the reduction of Ti4+ to Ti3+ causing oxygen vacancies at

high sintering temperatures. By adding small amounts of acceptor dopants with

ionic radii between 0.5 and 0.95Å, reduction during sintering was suppressed. The

upper limit of ionic radius was discrete with almost no observed effect using

dopants >0.96Å ionic radius. In addition, the microwave dielectric loss of undoped

TiO2 could be improved by annealing at 1500C for 10h in air, presumably as a

result of re-oxidation. High loss samples exhibited a dark ‘core’ to the naked eye which was absent in low loss ceramics. Transmission electron microscopy revealed

that grains in the dark core contained planar defects attributed to the condensation

of O vacancies onto specific crystallographic planes, in a manner similar to that

observed in Magnelli phases.

Dielectric Loss Caused by Oxygen Vacancies in Titania Ceramics. R.C.Pullar,

S.J.Penn, X.Wang, I.M.Reaney, N.M.Alford: Journal of the European Ceramic

Society, 2009, 29[3], 419-24