Available online at www.sciencedirect.com Journal of the European Ceramic Society 29 (2009) 419–424 Dielectric loss caused by oxygen vacancies in titania ceramics Robert C. Pullar a,∗ , Stuart J. Penn a , Xiaoru Wang a , Ian M. Reaney b , Neil McN. Alford a a Department of Materials, Imperial College London, Exhibition Road, London SW7 2AZ, UK b Department of Engineering Materials, University of Sheffield, Mappin Street, Sheffield S1 3JD, UK Received 6 June 2008; accepted 23 June 2008 Abstract Undoped TiO 2 exhibited deterioration in microwave (MW) dielectric loss as it reached its maximum density due to the reduction of Ti 4+ to Ti 3+ causing oxygen vacancies at high sintering temperatures. By adding small amounts of acceptor dopants with ionic radii between 0.5 and 0.95 Å, reduction during sintering was suppressed. The upper limit of ionic radius was discrete with almost no observed effect using dopants >0.96 Å ionic radius. In addition, the microwave dielectric loss of undoped TiO 2 could be improved by annealing at 1500 ◦ C for 10 h 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. © 2008 Elsevier Ltd. All rights reserved. Keywords: Oxygen deficiency; Defects; Electron microscopy; Dielectric properties; TiO 2 1. Introduction TiO 2 possesses a relative permittivity (ε ′ ) which is highly anisotropic in single crystals, ε ′ = 89 and 173 perpendicular and parallel to the c-axis, respectively, 1 but which averages to 100 in randomly oriented polycrystalline samples. Strong temperature dependence of the dielectric constant results in a high temper- ature coefficient of resonant frequency (τ f ∼ 450 ppm K -1 ) and although microwave filters have been constructed using TiO 2 , 2,3 it is generally considered to be unsuitable for this reason. Instead, TiO 2 is found as a constituent raw material in many temperature stable dielectric resonator (DR) compositions, e.g. Ba–Ti–O, Ba–RE–Ti–O (RE = Rare Earth), Zr–Sn–Ti–O (ZST), in com- posites with Al 2 O 3 , and doped CaTiO 3 . One problem of utilising TiO 2 as a raw material is that the Ti ion can exist in several valence states. This may lead to the presence of O vacancies (V O •• ) which are considered detrimental to the dielectric loss (tan δ), often expressed as a quality factor, Q, where Q = 1/tan δ. Typically in TiO 2 -based ceramics such as BaTiO 3 , 4 “coring” is observed; a term used to describe the dark, oxygen deficient interior of a sintered pellet. In order to combat oxygen loss in ∗ Corresponding author. Tel.: +44 207 815 7540; fax: +44 207 815 7599. E-mail address: r.pullar@imperial.ac.uk (R.C. Pullar). the capacitor industry, BaTiO 3 is modified with acceptor dopants such as Mn which inhibit the formation of Ti 3+5 in accordance with the defect equation, Mn 3+ + Ti 3+ → Mn 2+ + Ti 4+ . In rutile reduced in H 2 to form TiO 1.8 (Ti 5 O 9 with <1% Ti 6 O 11 /Ti 4 O 7 ), V O •• condense onto specific crystallographic planes which ultimately shear to create new structures, gener- ically referred to as Magnelli phases. 6,7 The space charge potential and the spatial distributions of defects in TiO 2 have been calculated and shown to vary with donor or acceptor doping. 8 In addition, Templeton et al. 9 showed that dense, high purity TiO 2 had a high dielectric loss, Q f < 6000 GHz (Q f = Q* resonant frequency in GHz). Furthermore, they demonstrated that doping with a range of divalent and trivalent acceptor cations with ionic radii between 0.5 and 0.95 Å greatly decreased dielec- tric loss, while other cations had little or no effect. 9 The low Q f associated with undoped TiO 2 was attributed to the presence of Ti 3+ and, as a consequence, V O •• , whereas the improvement in Q in doped TiO 2 was considered to arise from a similar charge compensation mechanism to that shown above. In this paper, the authors report the effect of 16 further cation dopants in TiO 2 which are compared with those reported previously. 9 The effects of sintering time and annealing were 0955-2219/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2008.06.019